If the fiberglass in that area is not painted, or can be sanded, you could epoxy
in some bracing material and hold it in place by inflating a basket ball (or other appropriate inflatable) under it.
The material you use could be a piece of 3/4" plywood
covered with glass, or a layup
of fiberglass u-channel over a carved foam core.
If its an opening to the bilge, I would not completely block it.