ADQdory-
Hi!
Sorry about your problem!
If I only needed a 5" x 5" piece of plywood, marine grade, etc., I'd buy a door skin, which is about 1/16" thick, cut it into 6" x 6" squares, then laminate them together using a waterproof glue like Resorcinol or even epoxy resin, being sure to alternate the grain/direction of the wood, etc. Clamp well. Cut the 6x6 down to 5x5 after it sets up solid.
If you weren't sure Phiilippine mahogany would be dense enough to resist the compression, then go with a more dense wood, like Birch, or something else appropriate.
One could even alternate layers of fiberglass cloth (like thin deck cloth) between the layers of wood for added strength, but that might be overkill !
You could even build a laminate such as that above, layer by layer right in the space it's needed, but the glue-up would have to be planned out and the pieces fitted beforehand, then glued and clamped all at once.
Alternately, some supply houses will sell you a partial plywood sheet, like a 2' x 2' section, or may have a damaged sheet they will sell you a section of.
A marine repair shop would very likely have some small sections available from the sheets they use to repair boats, as well.
I like to make special lay-ups / laminates for projects on boats. There aren't too many things more beautiful than something laminated up of a dozen or more alternating layers of wood, finished naturally.
Good Luck!
Joe. :teeth :thup