I also wanted to copy thousands of slides. I wasn't terribly concerned about image quality for most of the slides; I just wanted electronic copies. If I needed great quality for some images, my local camera shop can do that.
I have a Plustek scanner, but it was way too slow (about a minute per slide). I decided to try photographing the slides using my 100 macro lens on my 60D. I tested by simply hand-holding a slide to see if I could photograph it. I could, so I made the following fixture:
The slides were held to the left side of the center board (where the red oval is). I used 4 flat-head screws to create a place to insert the slide. To the left is a white piece of paper wrapped around the end of my fixture. Then I used a small LED reading light as the light source. I attached the camera by bolting the head of my monopod to the fixture. I used Auto White Balance on my 60D.
Using this, I could photograph 5 - 6 slides per minute. I did almost 4000 slides over the course of a week. Here is one sample. The original slide was taken in 1977:
After each slide was photographed, post-processing was primarily limited to straightening and cropping. I'm happy with the results, and I now have electronic copies of about 30 years worth of slides.
5D3, 7D2, EF 24-105L, EF 100L IS Macro, EF 70-200L f/4 IS, EF 100-400L, EF 100-400L II, EF 300L 2.8 II, 1.4x III, 2x III, 430EX II