hit a hardware/home remodeling/renovation stores, something like home depot or lowes
first thing that comes to mind are sheets of PVC

For my main rig Iv been running same ATX case without side covers since early 2000 😀 I cut it up in multiple places since then, PSU is sitting in former 5.25 bay etc
It still has the big green DURON sticker 😀 https://commons.wikimedia.org/wiki/File:AMD_D … cessor_Logo.svg

Yes I have used plexiglass sheet years ago, had to cut to fit (and then paint it as close to case color as possible if desired). Find the proper thickness might be a bit hard...looks like about 1/8" for yours ??
Your case: you would drill two small holes on the top frame then slip it in, the bottom of the case lip holds bottom and use the two screws to hold top in place.
You could try first with some packing box card board to see if it might work ok before buying a plexi sheet....

thank you guys