No. That is just glass matt. It will crack if you catch a small rock at speed, and would leave a VERY rough finish anyway. Tom suggested you create a mold which would be great but a lot more work than just laying up flat pieces and then seaming them together at the joints...also being inexperienced with fiberglass work, you would have a lot of trouble with bubbles at the corners.
I would really recommend using a nice, very smooth flat surface to lay your pieces up on and then trimming them to the size/shape you need and putting them together for your final shape.
Also a good alternative would be to make your box out of 1/4 inch plywood glued to shape and then putting a layer of fiberglass all around on the outside. Then you wouldn't hyave to worry about getting the pieces to release from the surface you laid them up on.