I'm now making a motherboard tray for ATX mounting from some old cases I've got laying around. Will post pics.
Making motherboard mounting tray for ATX in Mac case.
Here's an old case I've got laying around with an ASUS board.
Here's the mobo tray:
Drilling out rivets:
Top PSU mounting got in the way, so we cut it off with a hacksaw.
I'm going to work on the PCIE/GPU Slots next weekend.