Neat photo! Brown & Hauptner Marine (local SeaCraft dealer) once had a 21 in their shop with some strange steel reinforcement under front deck. Turns out it was the mount for a .50 cal machine gun! They suspected boat may have been used in same adventure as your picture described!
Meant to tell you, I had similar need for quick access to back of instrument panel. Instead of a hatch, my solution was to put a piece of piano hinge along bottom to allow entire cover to fold down; it's held by some clamps on side. Will send photo when I get a chance.