I can think of a way of retaining it, I don't know if it's feasable though.
have two M4 holes drilled each end of the block over the three lugs at each end, have them 'like' countersunk to accomadate springs to push the M4 bolts upwards. have two peices of metal (call them retention straps) that fits under the three lugs at each end with two M4 threaded holes in each one for the bolts(either side of the middle lug).
position the block over the top and thread the bolts into the 'retention straps', the springs should act as a buffer to avoid overtightening. it would be fidley but doable.
you could probly make the retention straps into a one~peice square plate assembly that fits around the socket. to aid fitting/hole alignment and make it more secure(stop it slipping over the lugs!), make one end so it slides in under the lugs for fitting and slides back out for removal. use springs or tiny finger nuts for tightening/fixin in closed position.
:[X]: for fitting
:[ X]: for removal
X= socket, : = holes for mounting, [ = retention straps.
Doh!, thats what the top pic is ain't it?. I thought that 'bottom bit' was holes you were gonna put in the mobo!!, not a peice under the lugs [hides head in shame] oh well great minds (and fools) think alike
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