I work for an ornamental plastering contractor and such molds are well
within our grasp. Well-supported, the mold would not need to be as thick as
5 times the thickness of the desired cast.

I have thought of making just such a mold but I don't think it works
economically for me. Plus I hate glass and resin work. It's worse than
Formica work. It would ideally be cast in epoxy, specifically a
vacuum-bagged, SCRIMP (vacuum-formed, glass-impregnated epoxy) technique.
This would be a very expensive proposition. It could be done as plain, old
polyester resin but if I were to take the trouble to build a mold that size,
I would go full epoxy.

the interior finish doesn't have to be an interior mold. One could simply
make a liner mold that allowed for some insulating space between outer shell
and inner shell. The two parts would be bonded together at the bottom edge
seam and various points along its section. It could be really cool but like
you said, John, a lot of work. The resin would also be pretty expensive.
Best to leave it to a proper plastic and glass shop, I think.
--
mordo
1990 Carat