Assuming you're not using the bulkhead as a foot rest, a single ply of glass that's waterproof is all that's required. Any strip built bulkhead properly glassed will be fine. Generally less stiff (again assuming you're not using it as footrest) is better since it can't create a hard spot on the hull/deck where it's mounted and any shocks those parts get won't be transmitted to the other side.

Bill H.