Create a form in the shape of your box, or build several forms that will fit together in one box. This form will fit outside the fiberglass box, the fiberglass mats being laid inside of it. Make the corners straight and tight. You can use foam, cardboard or even wood for a form.
Finish your form by sanding it smooth, and then paint it and finish it with Bondo if needed. You should always paint the Bondo to ensure that you can easily remove the form, so the fiberglass does not stick.
Apply your fiberglass strips and resin to the form. Lay them right over the form evenly at the same thickness. Layer or cross-layer the fiberglass for a stronger box. In some cases, a cross or diagonal layer will make your box stronger, but also heavier. Decide what is more important, strength or weight, and determine how the thickness affects your project.
Cut off the edges of the fiberglass strips to form. Make sure they are neat. You may also want to fold down your edges and reapply resin for a ribbed, thicker edge.
Let dry for several hours.
Sand your box using 80- or 120-grit sandpaper and working up to 360 grit. Make sure each edge is sanded finely and there are no gaps, cracks or shallow areas on the box.
Remove the form by pulling away the cardboard or wood. If the mold does not come out, you can gently tap the fiberglass box with a rubber mallet, or push lightly on the sides, twisting the fiberglass slightly and evenly until it pops off the mold. You might also want to tap the edges as you push and twist to dislodge the mold.