This is a contraption I designed to allow me to very quickly scan thousands of my parents 35mm slides. The slide drops in the small slot, put your phone in the back slot and have your phone display a white screen at full brightness (there are several free apps that can do this). Then use your dslr with an appropriate macro lens to copy the slide. I use a panasonic GH5 and a sigma 105mm macro lens. I set the camera to aperture priority mode at f8. I also enable the HDR mode in the menu to maximize the dynamic range. I white balanced manually on the lighted surface of the phone. Some film stocks looked better by manipulating the white balance a bit, but this provides a good starting point.
Personally, I mounted the contraption to a piece of wood and then mounted a manfrotto plate receiver at the other end. The manfrotto system allows about 3-4 inches of sliding front to back, so it's easier to get the slide to fill the frame at the given focus distance. The height can be adjusted by placing washers between the slide holder and the wood. Mine is about 1/4" up. And lastly, I put black gaff tape on the inside of a shoe box and covered most everything with the box to eliminate any stray light. I'm very pleased with the results and I can scan about 8-10 slides a minute with this method. The tinkercad link is below, this could easily be modified to accept different formats of film slides or negatives. I think the phone slot is big enough to accommodate most phones, but feel free to remix it to your needs.

This thing was made with Tinkercad. Edit it online https://www.tinkercad.com/things/jHZOsaaTYwF