good point...
*thinking*
here's one idea. not 100% guaranteed reliable, though. i'd try anyway.
mark your 8 or 9 points where you want the standoffs to be attached to. then, using a drill bit smaller than the standoff threads, drill into the wood at the 8 or 9 points. then screw the standoffs into those small holes which should keep them nice and snug. maybe you can find a glue to use in those holes to help... something that interfaces with metal and wood well. use as many standoff points as possible for increased strength.
i'll try to come up with other ideas..
oh, and i'd cut out the foam padding under the motherboard, too. no use in it being there and insulating. might even short something out.