Fiberglass strength

your friend is wrong. Go ahead and put one layer of fiberglass on your box and STACK the filler, and see how long it holds up :). Seriously though, the resin/matte combo is what holds up. 5 layers of that and you have a pretty solid box...its even hard to drill thru it. The filler is just for shaping and smoothing your piece. So next time you see your friend, give him a kick in the cherries...not a kick of cherry like the commercial says :)-

.....body filler has no structrual strength what-so-ever....

the strength only comes from the FG mat (for strength in all directions) cloth (for strength in 2 drections) biaxial cloth (also for strength in 2 directions)

the less resin used the better too...since it also dosent add to the strength either but only the weight of the project...

kitty hair / duraglass, will add strength...and a boat load of weight, and will also help prevent large flatter projects from warping...

ALL flat panels should be made from 1/2" or 3/4" MDF

Duraglass is a brand name ( like Kleenex ). You can ask for short strand fiberglass body filler and it will be the same stuff. Use long strand for larger areas and short strand for smaller areas.

Fiberglass matting has to be used as your building up the walls of the enclosure for structural strength. Anytime there's a curve the fiberglass is stronger but in the flats it's weaker. Duraglass is essentially a reinforced bodyfiller & adds a minimal amount of structural stability to an enclosure. Duraglass is used more for  smoothing out surfaces or filling in larger dips where strength is needed & where bodyfiller alone will not suffice.