I like the build overall, a few things I may do differently. One is put an old tealight within the mold to have it have a “resting spot”. Second, is maybe to add a slot at top to mount a picture frame of sorts. Oh and maybe make a more permanent mold (so you can make multiple of these).
https://www.instructables.com/Cement-Shadow-Box-Made-Using-PVC-Pipe-and-Cardboar/