I think that there are quite a few alternative setups that will give
good results with phone cameras, which have improved greatly. I've tried the suggestions by Lee S and by wheatiefan, and both
work, as do a variety of different permutations. So long as there's enough light and the coin and camera are stable enough, and the camera is far enough away to allow focus but near enough to allow details, it's pretty easy to take a reasonable photo nowadays. The photo I took and uploaded of my C.IVNI C.F.
Denarius earlier today was taken with a camera supported by nothing more stable than my wobbly hand, and against an unapproved background (not a grey card, not a distant background, but a random
bit of painted
wood).
Still, it came out ok. Most reasonable cameras and reasonable phone cams will take reasonable pics in reasonable light so long as one takes reasonable care.