fiberglass over bondo?

best way i find, if you have streched the fleece, is to strengthen the inside of the box, and then use a body filler to shape any creases or ripples in the fabric. mix up a batch of resin and fumed silica to a thixotropic paste and pour all over the inside of the box, the other expensive way is to bondo the inside of the box aswel.

Something else you may want to consider using when trying to build up a specific area is using dyna-glass (bondo mixed with fiberglass strands(bought that way)) there are other names for it as well, kitty hair etc, it comes in short hair, long hair and a version which you can't hardly see the hairs in it.  You can pick it up at most parts stores etc. 

But as the others have stated, get your glassing done, then you could use to build up the area you need to.  For buildup, this is a lot more effective than using bondo, requires less material than bondo and it is also less likely to crack like bondo sometime does when used to build up something to thick.

Something else you may want to consider using when trying to build up a specific area is using dyna-glass (bondo mixed with fiberglass strands(bought that way)) there are other names for it as well, kitty hair etc, it comes in short hair, long hair and a version which you can't hardly see the hairs in it.  You can pick it up at most parts stores etc. 

But as the others have stated, get your glassing done, then you could use it to build up the area you need to.  For buildup, this is a lot more effective than using bondo, requires less material than bondo and it is also less likely to crack like bondo sometime does when used to build up something to thick.