This did not go as well as planned! Being still a bit on the wet side
was good for fitting the two together and shaping, but attempting to
trim the bottom of the top portion resulted in that section sagging and
tearing. So we trimmed most of it out, leaving an internal ridge. We then
formed a slab and cut a circular piece to drop in against the ridge,
reinforcing the joint with coil top and bottom. It looks promising.