Can you access it from the inside-out?
Personally, if I could access it from the inside, I would plate off the end with a flat surface or with a pre-made fiberglass plate (using rivets to mount) and then wet-lay fiberglass over the plate from the inside...
If not... I would foam it up, shape it, and vacuum bag fiberglass onto it... You need a small compressor, the vacuum nozzle, bagging tacky tape, and the green bagging film.
Just wet-laying with the foam will not promote the fiberglass to sit properly unless a really low-weight twill is used (in which many layers are required and it is harder to wet-out the weave/cloth) and even then, air will most-likely build up around the edges and create weak-points...
-Gringo