Upload & Sell: On
My setup is made using the old Canon FD mount Auto Bellows and the slide copy holder that was made for it. I choose to not use the bellows part, but only the rail and slide holder. I then mounted a 5D2 body on a "spare" base part from another Auto Bellows, and that allows the camera positon to be moved smoothly and locked. I had to build an adapter mount, and get the camera at just the right height to align with the center of the slide copier. I then use my Canon 100 mm macro lens to shoot. I use a Canon ETTL flash on an off camera cord for rear lighting, and let the ETTL do the exposure. By shooting RAW you can do post processing for color correction and also adjust contrast lower to get reasonable looking copies. I normally just shoot the whole image area including a little bit of black around the edges, but if I want I can adjust the camera position closer and crop out part of the slide. The slide copy holder for the AutoBellows allows you to move the copy stage around, so you can crop in to any part of the image, not just the center.
It is a fair amount of adapting, and kind of expensive, but I really liked my approach where I can use an auto focus and auto aperture macro lens and ETTL metering. The auto focus catches different slide mounts and different amounts of bow in the slide, and the foucs is done wide open. And with flash lighting, I get consistent color balance and plenty of light to stop down and keep enough depth of field to accomodate the bowed slides. You do also need some ambient light on the slide holder difuser glass for focus and for live view, but the flash overpowers the ambient so it does not cause any color shift issues. I can bang away as fast as I can put the slides in the holder and check gross position via live view, and can do batch processing on the RAWs to get them to reasonable viewing conditon. I can then work on specific shots I like and do careful post processing croping, or even clone in the rounded corners made by the cardboard slide holder if I "need" the original compsition.
The old Canon Autobellows are a little hard to find at times, and they are not dirt cheap. KEH usually sells them for around $160, but they now do not seem to have them very often. The style of the rail and the whole autobellows was the same or very similar on many camera brands back in the day (1970's), so you may find a different brand bellows that has a good slide holder. My approach does not care about the lens mount, but if you want to also use the bellows for other uses or use a bellow lens for slide copy work, you can get an adapter to mount an EOS EF camera on the Canon FD mount of the bellows.
But if you do not want a DIY project, this route is not for you. Hopefully someone will give you a lead on a standard product that you can use, and I look forward to any information that may show up here, because I have a friend that is in need of a similar setup.