You can cut access doors in the bench, just make sure you cut only in the flat parts of the bench (not at any of the angles) where there is a single, thin layer of
fiberglass without reinforcing members and leave a 1-1/4" rim of
fiberglass around the top and sides of the cut. Once the opening has been made epoxy in some 1" wide strips of 3/4" plywood to reinforce the top and sides.
The problem with this approach is you either wind up with a really small door for accessing a big, big space, so you might try an alternate approach:
putting a hatch in the outside wall.