Clamping the body and block on a sloped neck joint

We're working on a bass uke with a sloped neck joint, somewhat like the hoefner bass played by Paul McCartney. We've got the neck block all cut out and sloped and the sides bent. They're very close to a perfect fit on the block but may need some very minor correction depending on how the clamping is to be done.

Anyway, the problem is that with the sloped joint we haven't figured out a way to clamp the sides/block together so that there is good pressure on the joint without anything slipping. Any ideas?

EDIT: By the way, I'm thinking the same issue would come up when you clamp the sloped side of a cutaway neck joint.

I'm not sure what your joint looks like. Maybe a photo would help?
Sounds like a job for waxed paper. When I do scarf joints on the neck I clamp both pieces down to the bench before clamping the glue joint. The waxed paper keeps it from getting glued tot he bench. Maybe you could clamp your neck block down tight and then clamp the sides to it.