After dicking around with all kinds of half-assed mounts, I finally settled on the A-Alpha helmet. Very light, fits giant melons great without "tight" spots, doesn't slip around and looks cool. I bought a new Rhino Mount on the EE for $50, a J-Arm, also on the EE for (I think) $25, and a new Norotos-type shroud from some Airsoft site for $12. It actually works good, is made out of aluminum and fits the Rhino and helmet great. I initially tried bolting the Rhino plate to the helmet (one bolt) and it worked, but didn't look right.
I tried every friggin' thing I could to rig up a mount, but it was FUGLY, worked like shit and wasted a lot of time and money. In the end I just bought the stuff listed above. Works great and looks professional for about $170 bones total.
You could probably do it even cheaper if you shop around. There's always mounts and parts on the EE.