I did something similar a couple of years ago and just as Ian, I have surprisingly good results.
I used a cold light viewing screen. On of those you use when comparing slides.
I have that flat on the table. Then I made a holder where I could conveniently drop the slide in and that would block light not passing the slide.
The holder straddles the screen at a distance above so any specs on the screen are out of focus.
The contraption then carries a similar drop in holder for an old biggish lens (Heliar 30cm 1:4.5 for 24x30cm negatives) and a mount for the camera.
The advantage lies in the quick exchange of slides and in convenience since I do not have any fold down screen. The long focal length of the heliar gives me plenty of space to maneuver the slides but I must of course use the camera in telephoto mode.
Gravity can be a great help sometimes

Unfortunately it does not work all that well with negatives. The negatives have a different gradation and further there is the orange mask. I have yet to find out how to process the images. mere conversion does not give sufficiently good results. My dedicated (= s---l----o----w

) scanner handles these problems reasonably well.
Göte
(On request I could post a photo but we all have to use what bits and pieces that are available)