the problem is mate, that you would have to make it in two halves around a plug to remove the plug, and then you would have to join it up again and being so small it would be virtually none feasable to an amature builder. You could however make a small box from 2mm ply, and coat that in grp with a final top coat of gel for water tight ness, and then make the opening and lid for it.......it's the only way at that size I could think around the problem, but the wooden box would remain inside the grp bnox for eternity.
neil.