The problem with placing something above the neg glass is that it does need to be stiff so it won't buckle with heat. Some neg carriers, like pro
Durst in particular, have built in registration pins. So you could use these for a piece of plexi or white polycarbonate with drilled alignment holes, and achieve approximate register. True register is a different subject, if you ever decide to get into lith highlight masks etc. Masking is a lot of
fun if you like hand-on darkroom work in general. There are soooo many ways of doing it, and various options to explore, even with a low