Now that you have the enclosure fleeced with resin on it you can lay down some more fiberglass.
1. It will be alot easier to fiberflass the outside of the enclosure. Use fiberglass mat to get more strength and thickness. 7 layers is the universal guide to how much fiberglass to use. It must be strong enough to not flex or bend in any way. I stand on mine all the time and it doesn't flex (I weigh 150lbs.) If you can stand on it w/o it breaking or flexing then it is probablly stong enough.
2. Once it is strong put a layer of fiberglass cloth over it to make it smooth so you wont have to sand it as much or bondo the crap out of it if you plan to paint it.
Anothr way to make it stong if use supports inside of the enlosure. I used wooden dowls to help strengthen it. Just cut them to length nd use some hot glue to hold them in place. Fiberglass them for best results.
Also A look at the amplifier rack I mae out of MDF and fiberglass. [ click to enlarge ]