Doc - If you haven't thought of this already, how about making the board and backing slightly oversized, apply clamping force only to the dimensions required, then grind off the excess (the outer, un-clamped portion) after curing? This would eliminate the need for extreme precision, and the problem of the clamp surfaces bonding to the work.
A couple of small wooden blocks could be fashioned to roughly the size you'd like the final result to be. Sangwich (as they say in Noo Yawk) the work between the two blocks, then put the blocks into the clamp. Any mechanism with parallel jaws could provide the force... vise, crescent wrench, etc.
Speaking of sangwiches, anyone wanna' know of a good, low calorie sangwich?
A half a sangwich.
Alas, poor diode. I fried him well.