If you have a 3D printer you can design and print one. Did that many times in the past. There are also a lot of pre designed models available on Thingiverse, Printables etc.

Else you can look at Aliexpress or so for a closed one and cut the holes yourself. Or try one from another motherboard.

Many backplates from roughly the same era fit with several motherboards. I have used same IO plate with different motherboards.

If you can access 3D printer, that is one way. You can find designs in printables or thingiverse. There are some that are designed specifically for certain MB, but then there are blanks and meshes, where you can cut or CAD your own holes.

I buy spare backplates from Ebay. You just need to find one that matches the layout of the motherboard components. Most of them are pretty standard.

This is a standard ATX layout which all early ATX motherboards were shipped with, so any standard plate will work.

If you go the 3D print route, I have this one in my collection:

I've been lining the backs with foil tape lately and it makes them feel more realistic (and actually shield).

Shponglefan wrote on Yesterday, 19:25:

I buy spare backplates from Ebay. You just need to find one that matches the layout of the motherboard components. Most of them are pretty standard.

These are indeed some of the most common backplate, not too hard to spot, even in a large pile.

Thanks for the suggestions everyone. Sadly, I don't have access to a 3D printer. Yet. It is something I'm interested in though as I do character modelling and sculpting. If it is fairly standard and easy to find, that's good. Reassuring. I'll take a look on eBay and AliExpress and see what I can find. 😀