Any ideas of how to fabricate a motherboard stand-off thingie??
If you don't have a tap-and-die set to thread your own screw holes for an actual standoff, mark holes for your ATX motherboard mounts on the sheet aluminum, and drill holes straight through.
the holes should be big enough to allow a 1" machine screw to slide through. the screw should come in from the bottom, and sit head-down. tighten the screw to the plate with a machine nut.
you now have posts which should hold up the motherboard. to prevent the mobo from shorting to the plate, cut small pieces of vinyl tubing (aquarium air line should work) and use the tubing as standoffs to raise the motherboard above the aluminum plate.
thread the motherboard through the screws and double check the reverse is not shorting on anything conductive. use a second machine nut to secure the motherboard on the DIY standoff. you don't need to, but if the screw body is too tall take a dremel to it and lop off the extra height.
i have uploaded this poorly lit photo to help. the motherboard rests on the clear plastic tubing, and is fastened from the top by a second machine screw. this was from a modded power mac converted to an intel mac. it's not pretty but works in a pinch.