I've designed an printed compact non standard sized 4x5 holders for my Polaroid cameras and they work fine without light leaks. They are not QUALITY film holders, but are FUNCTIONAL. You can't pull the dark slide out completely, and they are made for 4"x5" negatives, not 3.9"x4.9" ones. You can check out how I did it at
https://www.thingiverse.com/thing:5254398 --I started out by printing just the rails and building it out of mat board. Later, I designed the whole holder, though it takes a good 8 hours to print.
For a dark slide, I started with using bristle board, which worked fine with paper negatives, but wasn't light tight for high speed negatives. I switched over to using black plastic casing from a file folder. It slides easily and keeps the light out with ISO 400 film.
This just to say that it can be done on the 4x5 scale. I'm going to try designing and printing 5x7 holders someday. I think that should be possible.
But 8x10? One problem with the Ender 3 V2 (which I have) is the bed size. You can print up to around 8.5" in all dimensions, so you wouldn't be able to print a full sized 8x10 holder without expanding the printer at least in one dimension, which can be done but costs more $$. Of course, it probably wouldn't be worth it to purchase a printer just to print up a few film holders unless you're planning on using it for other things. From personal experience, I can't imagine life without a 3D printer...
I managed to make a double sided 8x10 film holder out of wood using just a 2x4, 1/2" plywood, and black mat board for the dark slide. You need a table saw to do this. It's not a QUALITY holder, but it does work.
I'm thinking that 3d printing an 8x10 back would be impractical because of the amount of material used. I would think it would be better to build something like that out of wood. Be aware though, that the back of a large format camera is a little tricky to build.