I was thinking of doing a similar thing a while back before deciding that it was cheaper (time and resources) to just buy it.
BUT, while I was gathering info, I figured I would make the top of the block from Lexan (see through and would look nice with a couple of blue LEDs drilled into it). What I was thinking of doing was to use the 4 holes that AMD mobos come with. This is not as universal a solution as you wanted, but its the easiest way of mounting and dismounting the block, and its pretty secure this way.
Since the holes are pretty standard, all you have to do is measure them and make the top longer than the block so you can put some holes in the top and screw it into the mobo.