That's brilliant, John!

Other than the procedure you describe I'm slightly stuck on how to
fabricate the back brace and get your hand back out. I've been
thinking the back (-of-the-hand) brace would need to be added later
somehow.

For instance, imagine you've got a nice bucket of papier mache. Make
a blob like the one in the photos section of this news group, but take
more care to smooth away the finger trenches. Let it dry. It will
shrink a bit but that's good. Now smear some grease on it or cover it
with clingy wrap, or something, and mold a shell of papier mache on
top of the blob. Or use epoxy or the water-activated fiberglass. When
that hardens you've got a thin, curved surface that fits your palm.

But it still needs the back brace. Since I've been fiddling with
wires it seems natural to apply one to the shell somehow and then bend
it to fit. Or bend 1st, apply 2nd. But how to attach wire to shell.

Maybe my original blob had a trench such that the shell has a boss
that can be drilled for the wire.

That water-activated fiberglass --- is that what doctors use to make
casts for broken arms, etc.?