Good idea; slug of urethane, separator piston, another slug, etc.
The parts (are supposed to) have a tolerance of +/-0.002" on length so I do not need much deformation in the urethane. It would be nice if I could accommodate the occasional piece that varied by 0.020". These are small parts parted off from barstock and occasionally a chip sticks to the end of the stop which means that part may be short. With our current fixture these parts sometimes spin and break the cutter in the mill.
Your experience with the urethane pressure distorting the fixture is something I considered. I plan on using honed hydraulic tube with the part clamping pistons actually protruding through holes bored through the side; this, I hope, will remove the need to deburr the inside edges of the side holes because the urethane will not actually flow into the hole. The hydraulic tube will be clamped in a split block to support it both axially and radially because it is going to be severely weakened by the side holes. While typing this I just realised if I make the ends of the side pistons hemispherical the urethane will ba able to flow past them more readily.
One thing I need to determine is what kind of psi will need to be generated by the urethane. Our current fixture for these parts has a clamp straddling two parts held in the center by a 3/8"-16 cap screw tightened to something like 8 or 10 lb-ft. I need to get some idea of what force is being applied to each part with this kind of bolt tension.