WAY too much resin. When there is too much resin in any fiberglass piece, the strength the fiberglass WOULD have slowly decreases, as resin is very brittle by itself. The lack of reinforcement makes cracking almost inevitable.
Also, you should do all of your fiberglass layers in one sitting, and use MINIMAL resin, just enough to wet the mat out. A roller helps with this. If you absolutely MUST do your layers in different sittings, sand the dried layer before applying more mat.
What weight mat did you use? What brand/type of resin? How many layers? I would be very skeptical about using alot of power in that box. I would add layers around the insides of the rings for extra strength.
Edit: Lot's of air bubbles, as well. These are weak points in fiberglass, grind them out and fill them with glass (correctly) or with a fiberglass reinforced body filler, like Duraglass or Kitty Hair.
