SG Customs build the "Mesh-ATX" in the SSUPD Meshlicious using a fullsize ATX mainboard, which inspired me to try my own version of it, but with Micro ATX, hence "Mesh-mATX".
Part #1 of his build:
Part #2:
Another option that comes to mind: Using cases that are technically meant onky for mATX, but could in theory support ATX, if one would position eg. the PSU differently, and then either flipping them on the side or back to make them sit properly in the Kallax shelf, while still being able to access the IO panel. Those are mostly traditional structured cases, with the PSU on the bottom (ie. modern layout).
One of such cases would be the Itek SMALLCOM-P / B. You could just flip the PSU like its done in the Meshi, and add a few more mainboard spacer, then a regular ATX board should fit perfectly well.
So, thats it for my insights for now.
cu, w0lf.