I tried with tinkercad, and I struggled. I was again able to align the shapes directly on top of one another, but when it came to modifying the actual shape I could not figure it out- is there a tutorial on adding shapes together or anything, or even carving stuff out?
I looked and couldn't find anything this specific. It seemed like a much easier tool to use, but I couldn't figure out how to do what I wanted. I also used
https://www.thingiverse.com/thing:4935721 (right cover stl) and https://www.thingiverse.com/thing:5704716 are the two files I'm trying to merge. I'm gonna try playing with windows 3d builder as someone suggested as well