Re: Cockpit drain holes
Good luck. I've got a Contessa and I do think drains could be improved upon. I can speak from experience taking a wave over the stern and then sitting in a bathtub staring at my underwater engine panel for a few minutes. Of course we had the hatch boards in otherwise it would have all drained nicely into the cabin - one weakness of the Contessa is the big low cockpit and non existant bridge deck. I'm guessing your boat isn't too different.
But it's a hard job in an area with little room. One thing to consider would be just to double up the system you've got with two more drains and two more through hulls. This avoids the risks you'll encounter if you try something new (for example I agree with the problems mentioned above for your fiberglass idea).
If I were doing that I'd put the through hulls a few inches higher than the existing ones which are hardly above the water line.
1979 Contessa 26