Thanks, Bethe, everyone. I think I will look at glass carriers, actually, although I appreciate that if I'm careful and the film isn't too curled, that I might be able to get away with a 6x6 carrier and some clever masking.
Yes, I do have the "non-Carl Zeiss" Holga (!). But actually, I probably have just as much 35mm from pinhole and homemade cameras, where the exposure also cover the edges. I uploaded an example (Tri-X, in this case, from a homemade pinhole camera) to show an example of what I'm trying to print.