How much Fiber glass would it take to cover a Decent size Square sub box? I have never seen it in huge amounts of fiber glass. I figure 3 layers and then Bondo. Also how do people does those all custom Fiber glass boxes.

 

Does the box already exist or are you making one out of fiberglass?

Custom fiberglass boxes= tools, $$ and time. Large flat areas (usually the base) are made with wood, fiberglass for the curves. Chicken wire used to create mid-air shapes for fiberglass to stick to. Metal braces to suspend wood (usually baffles) in mid-air. Layers of cardboard or 1/8" wood used to create raised areas flush with mounted items (amps, x-overs, etc). Lots of bondo and sanding to make everything smooth. Then 3M Super 77 adhesive for carpet/vinyl to stick or a good paint shop to paint it.

Have fun.

Boat repair shops sell fiberglass real cheap.

 

Do yourself a favor and check out fiberglassing/car audio installation books at amazon.com.