Marco 1 1/4" thick frames are common and they are standard. What
size opening are you looking for. You need not cast this. You can
use the following method.
Depending on the depth you need take two or more mold frames with
same size openings you could stack these on top of one another. You
have to make sure that these do not move or shift and that is
accomplished by drilling two holes and put a pin through each of
these holes. ( In fact there are mold frames made of two parts that
you can buy but these are made for 1" thick only, you can stack
these together to get 2" or use one piece less and get 1 1/2")
Regular 1 1/4" mold frames come with a 2 1/2" x 3 1/2" opening.
As a last resort I would try the pipe. A 4" or 5" OD Aluminum
tubing comes with about 5/8" thick wall and strong enough side walls
to be used as a mold frame. Castaldo use to make rubber precut in
round disc for the European market.
All the best.
Kenneth Singh from New York