This is non of my business but do you have warm hands?
The reason I ask is because I do and I find it difficult to work
with wax for this reason, the wax tends to warp because of the heat
If you want to make your shallow box out of metal you might want to
consider using brass profile strip to build a model that you can
then makes a mold directly from. Try a hobby shop for the profile
strip, there is a company K&B which sells brass profile strip used by
model builders. I don't know of a website that shows these profiles,
the K&B website only shows the displays they offer for wholesale.
You should have no problems finding 90 degree angled pieces that you
can use to build up your shallow box frame as well as strip and sheet
you can use to build up the second portion of the shallow box.
I am not sure where you are but places near me where I find K&B
products are, Industrial Metal Supply (the best!), OSH (mostly tube,
wire and sheet), Do it Center (almost as good as Industrial Metal
Supply, but more pricey).
May I ask, what are you making?
I hope I have been helpful