I got my design and print tasks sorted out and now have liners for the top and bottom of my glasses case made out of rubbery TPU:

That serves to reduce the excess space and cushion any movement my clip on specs do make. It also adds a nice overall heft to the case. It did take quite a bit of measuring and thinking to figure out how to create the lining but I was pleased not to have to go through lots of test prints. The resulting fit isn’t completely perfect but safely past “good enough” so I’ll count this project as done for now.