Ah now that is the tricky bit.
I have a short piece of aluminium bar (40mm x 4 mm) attached to the top of the tube rings. I have a hole drilled in the bar and the screw goes through the bar into the bottom of the manfrotto. Works a treat.
I'll take a photo and post it.
Edit: Here's one I prepared earlier. 
It also has the advantage of making the rings a bit more rigid as well which is a bonus.