I've been making custom boxes all year from plywood, having sourced an obscene quantity of CNC off-cuts - all precisely 29.75" x 13.75" x 11mm. This stuff is 9-ply with paper-thin surface veneers - so could also be described as 11-ply if Afur Daley was flogging it.
But - as Hivemaker described - this stuff ain't marine ply. Far from it. There are visible voids and even overlapping plies which show right through to the surface. But hell - I'm making bee-boxes, not a boat or pieces of furniture.
The first thing I did having produced an offcut, was to boil it up in a kettle for a good 2 hours. The glue held ok, but some overlapping core plies near to the edge became swollen to the point of forcing the laminates apart. That told me in no uncertain terms that any exposed edges would need to be sealed, and sealed well.
I've made bait boxes from this stuff, although they turned out to be a bit heavy for hoisting up into trees, and by screwing 2 planks together to produce a 22 mm wall thickness, I've made a rather useful Long Deep hive which takes 14x12's as well as Top Bars. Currently this houses an August swarm on frames with an overhead feeder (in a National Super) supplying syrup and fondant.
Right now I'm in the process of completing the first prototype of a double (2x5) Top Bar NUC which uses either Top Bars or modified (trapezoid) Hoffman Brood frames, using a single thickness of this plywood. By removing a divider this converts to a 10-frame/10-bar NUC so I've fitted a mesh floor (closeable).
Also on the agenda are some mating NUCs made from the same stuff, using 1/2 width brood frames.
So - with a sheet of good quality ply, I'd suggest making-up some custom stuff, rather than producing copies of 'standard' boxes which can so readily be sourced elsewhere.
Good luck
LJ