If you're going to go fiberglass with the cheap polyester resin...
Make your box out of wood and get the shape you want. Then coat it with fiberglass mat and resin. Make sure you use enough resin to get all the little air bubble out. Most of the work is getting as much of the air as possible out. Let it kick and then go to town with 60 grit before it gets really hard. You can knock a lot of material off without too much work that way. Doesn't get all powdery either. Stop and wait if you see the fibers tearing out of the soft resin. Need to wait until it's just a little harder. Should be where you can leave a slight mark with a strong fingernail.
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