How can I soundproof a hatch door in the floor?
1.5" X 2 feet X 3 feet or so?
That's not very thick to support 150+Lb without flexing, which'll give you a drumhead effect regardless of how much foam is used.
Consider putting an angle iron stiffening frame on the underside of the door.
Attached firmly, it should eliminate flex, and most of the drumming.