I think the 4-chip pod is a more logical target. It should be possible to make a single board which fits both the Gridseed and U3 heatsinks; I'll have to get ahold of an RBox pod to see what it'd require but I know I can hit the other two for sure.
CanaryInTheMines used to sell them on Eligius iirc, maybe they might have one or 2 (dead/alive) to spare?
if not, I will look out for one or 10 for you, if I get my hands on one I'll PM you.