If you aren't making some sort of curved shape there is no need to fiberglass anything.
Spray a few coats of automotive primer on the box and then sand it with 180 or 220 grit paper. Then spray another two or three coats on and sand that with 400. It's best to sand with a block but if you don't have one, sand in a circular pattern to avoid cutting grooves with your fingers. The grooves may not be noticeable in the primer but they will in a glossed finish. Oh and, a short piece of wood makes a suitable block.
Once you have the primer sanded smooth with 400, paint with basecoat clearcoat or with a good enamel from a buzzbomb. And if you happen to sand through the primer ANYWHERE, spot prime it lightly before painting because the raw wood will soak up the paint and make the finish dull.