I should be able to borrow an e38 ecu locally. Looks easier to make a mount for it than the ls1 ecu. Just let me know how the e38 needs to be orientated (pins up or down, if up, how much clearance needed, etc).
The hex head bolts in the pic are actually 5/16" lag screws that go into those plastic pieces sticking down from the evaporator box. Feels solid - couldn't think of another way to mount it that didn't use a big bracket that would require separate prints. The ls1 rear mount was hard to install since the lag screws wanted to sit directly above the ecu and the cover bolts were almost impossible to get to with it in place. Too lazy to fix the design now since it seems to work.