Starizona [link] produces the HyperStar, which converts a Schmidt-Cassegrain telescope into a DSLR mountable Fastar by placing itself with the camera mounted in the conventional position of the secondary mirror. The HyperStar is available for Celestron Fastar compatible models, Edge HD series and for Meade standard SCT models like the LX90 or LX200GPS (not compatible with LX200R or ACF models). There are also HyperStars dedicated to Celestron C6 and C8 as well as Meade 10" available, but not compatible with DSLRs.
 
Older Celestron C11 and C14 (missing the Fastar compatible label at the secondary mirror assembly) and older Meade 14" telescopes (with non-removable secondary mirror housing) need permanent modification (not applicable for older C925 models). Conversion sets are available from Hyperstar as well as from third party vendors.
 
The camera is mounted via adapters, which are available for T2 as well as bayonets like C/EF. The max. diagonal sensor size supported without vignetting is 27mm for all models, though a maximum diameter of up to 42mm is possible at the cost of more or less vignetting, depending on the sensor size (vignetting with full frame cameras is claimed to stay in acceptable limits). HyperStars provide an internal 48mm (2") thread for filters.
 
Attention should be paid especially if buying a used HyperStar: Not all older models are compatible with DSLRs. In 2011 Starizona states compatibility for all models produced since 2007, which can't be right since #HS-C14D replaced #HS19-14 in 2006 and the #HS-C11 was not replaced until summer 2008 by #HS3-C11. Well, take it like this.
 
Some more technical specifications:
 

• Hyperstar 3 for Celestron 9.25" F10 SCT [#HS3-C925]: MC, results in 235mm F2.3, Size=109.2 x 101.6mm, 817g
• Hyperstar 3 for Celestron Edge HD 9.25" F10 SCT [#HS3-C925HD]: MC, results in 235mm F2.3, Size=109.2 x 101.6mm, 817g

 

• Hyperstar 3 for Celestron 11.0" F10 SCT [#HS3-C11]: MC, results in 560mm F2.0, Size=114.3 x 109.2mm, 998g, 2008
• Hyperstar 3 for Celestron Edge HD 11.0" F10 SCT [#HS3-C11HD]: MC, results in 560mm F2.0, Size=114.3 x 109.2mm, 998g
• Conversion Kit for older Celestron 11.0" F10 SCT [#HSCK-C11]: needs an optional modification itself if applied to even older orange tube SCTs, 2008
   
• Hyperstar for Celestron 14.0" F11 SCT [#HS-C14D]: MC, results in 675mm F1.9, 2006
• Hyperstar 3 for Celestron 14.0" F11 SCT [#HS3-C14]: MC, results in 675mm F1.9, Size=154.9 x 124.5mm, 1590g
• Hyperstar 3 for Celestron Edge HD 14.0" F10 SCT [#HS3-C14HD]: MC, results in 675mm F1.9, Size=154.9 x 124.5mm, 1590g
• Conversion Kit for older Celestron 14.0" F10 SCT [#HSCK-C14]: needs an optional modification itself if applied to even older orange or black SCTs, 2008

• Hyperstar for Meade 14.0" F10 SCT [#HS-M14]: MC, results in 700mm F2.0, 2006
• Conversion Kit for older Meade 14.0" F10 SCT [#HSCK-M14]: removable secondary mirror housings have 6 screws, where as non removable housings only have 3 screws - the conversion kit is needed in the second case, 2008

Notes: SCT is not a series but an abbreviation for Schmidt-Cassegrain Tube. Fitting Celestron series are CGE (incl. Pro), CGEM (incl. DX), CPC (incl. Deluxe & GPS) and EdgeHD, available in standard Optical Tube Assembly with aluminum tube (OTA) and carbon fiber tube layout (OTA-CF).
Last archive.org capture is from 06.2009, when the C925 and all HD versions where still not released. Following the undated news history on their website the C925 came sometime in 2010 or early 2011 together with the Edge HD versions for the C11 and C14.
 

Tonta Electro Optical in Taiwan introduced their Lens2scope at the Photokina 2010. Use it to transform any C/EF mount lens into a spotting scope. It's compatible with EF-S, alternatively also available for N/F and P/K. The concept is by no means new - eyepieces for T mount lenses exist since ages - but as far as I can tell this was the first available eyepiece featuring a C/EF mount.
 
Lens2Scope 7mm Wide 1:4 (2012):
 

• F = 7mm F4.0 roof prism construction
• field of view is 60°
• magnification is 1/7 of the lens' focal length, 36x if the host lens has 1:1 macro capability
• eye relief is 17mm
• features a detachable tripod mount (for lightweight lenses only - 1000g at max.)
• Straight Eye Relief model size: 123 x 78 x 90mm, weight of 245g
• Angled Eye Relief model size: 125 x 78 x 106mm, weight of 225g
• both are available in black (#LS7-CESB and #LS7-CEAB) or white (#LS7-CESW and #LS7-CEAW)
• not compatible with so-called drive-by-wire lenses (Canon E-M and probably Power Focus)

Lens2Scope 10mm 1:4 (2010):
 

• F = 10mm F4.0 roof prism construction
• field of view is 42°
• magnification is 1/10 of the lens' focal length, 25x if the host lens has 1:1 macro capability
• eye relief is 20mm
• features a detachable tripod mount (for lightweight lenses only - 1000g at max.)
• Straight Eye Relief model size: 180 x 90 x 80mm*, weight: ~185g*
• Angled Eye Relief model size: 180 x 110 x 80mm*, weight: ~185g*
• both are available in black or white (#LS10-CESB and #LS10-CEAB) or white (#LS10-CESW and #LS10-CEAW)
• not compatible with so-called drive-by-wire lenses (Canon E-M and probably Power Focus)

*The 2012 Photokina material gives different specs here: 135 x 78 x 90mm at 205g for the straight model, 130 x 78 x 110 at 185g for the angled model.

Kipon now also offers a low budget solution here with their Canon EOS dedicated Kipon EOS-Tele 10x0 spotting scope adapter (2011).

 

Frii Designs TriLens (2017) "[...] safely carries up to three of your favorite lenses and changing your glass has never been faster. We're launching on Kickstarter the 9th of May [...]". Comes with belt clip, carries up to 100kg max. and rotates around its center, so the heaviest mounted lens will always go heads down. Made in Sweden.

GoWing Lens Holder (introduced in 2013 as Easy Lens Changer, mass production started in 2014) is a lens carrier with shoulder strap (belt clip version announced for 2015). It consists of a flat ring with mounts on both ends and a seperating bottom in between to keep the dust away from single side mounted lenses. available for Canon EF among others.

Hanse Inno Tech Compact Collimators 1 & 2 (2017) aren't for everyone. They are used to measure optical components and mechanical deflections to determine characteristics like backfocus. Quite usefull, if you have your own film studio equipment and are busy changing lens mounts from PL to EF to PL to EF all day long, run your own lens rental company or repair shop etc.. As a light source a Schott KL 2500 LED is used. Mounts available right now inlude Canon EF, PL and Sony E, more are announced to become available in the future.