I think this is a pretty efficient way to make shields assuming you make them 4 at a time.
-1 sheet of 4'x8', 3/8" plywood. Cut into 2'x3' squares. Yielding 4 cores.
I would prefer to bevel the bottom edge because I believe it is a more efficient design, but it would just be more cutting. After these shield blanks are cut, that would leave 1'x8' of plywood left that could be made into bucklers for spearmen or archers...
-1 roll of blue foam is ~2'x6', so 1 roll would make 2 layers on the front of a 2'x3' shield core. One more roll to cover a third layer on the front, leaving a half roll to cover the edges. Total of 2 rolls of foam/shield.
-x4 tee nuts, x4 1/4"x3/8" bolts, x4 1/4" washers.
-Strap it how you like, the Army surplus on I-44 sells thick nylon strapping $.77/ft.

I need a new shield, so if three other people want to help buy a plywood sheet, hit me up.

We could likely use a few shields in stock for loaner weapons, too. Maybe some due money could cove the remaining 3 people?

i'm considering it, do you know how much a core would cost?