I’m planning to build a coffin bookshelf for my niece.  Since I already had the bookshelf Sketchup plans, I turned it into a small 3D model.  The result is a simple partitioned coffin box with lid that my niece is using to store jewelry and other small items.

I’ve printed with PLA and PETG (no supports) with varying amounts of shrinkage but that shouldn’t affect the fit.  If you change the size, you may need to play with the lid size to get a good fit.