If you have a DSLR with a "medium range" zoom lens, this works very well and does not cost a lot to make.
Make a sliding camera mount (1 x board with hole for a 1/4 - 20 bolt for tripod mount on camera), make a "box" of matte black poster board (WalMart), line up the slide placemount and an opening in the back of the box on the same center line of the camera (opening to match the size of the slide), put a "lightly frosted" clear plastic sheet over the opening (sanded mine lightly with 600 wet paper - need to diffuse the light), get a coffee can, put in a ceramic (china) lamp holder, mount the can on the center line of the lens and get a "daylight color temperature" twisty bulb (keeps the heat down so you don't melt the plastic and/or the slide) and insert into the coffee can.
Take the cover off, put in a slide, put the cover back on, adjust the camera placement, zoom length, focus (use manual focus) and take a high resolution picture of the slide (best to use a remote shutter release). Load the picture into a photo imaging software (I use PhotoShop Elements), then crop as required and do post processing.
Lot more time consuming to describe than it is to actually build and do the slide copy. I did about 7000 slides about 9 years ago with this. Boring for that many but they all came out great, especially considering some were from way back in the 1960's and some that I had actually developed and mounted while in Vietnam in 1969.