Having thought about this a lot... the strategy shouldn't be to push down on the raft. This just results in pushing down on the bed and pushing down on the bed screws up the z-offset, resulting in layer separation at some point. New strategy... prevent the raft from rising by restricting its upward movement with a closely-measured clip. So my new clip design is basically an adjustable pair of arms with clips on both left/right side of the bed and bed plate. Three of these clip assemblies will be added at the beginning of the print job, initially adjusted so that the raft won't get in their way. After the raft has been completely printed and the 1st layer of the part is being laid down, I'll then adjust one cam per clip assembly to adjust the clips inward, while applying a little upward pressure to get them to sit on top of the raft on left/right sides. Everything will be created with such tight tolerances that it should just fit in the vertical direction. This should arrest any upward curl of the raft and therefore that of the bottom of the part itself. Since it doesn't push down on the bed, it shouldn't affect the z-offset for any of the layers. Here's the STL of the key/cam assembly that I've done so far. It's printing now.