Hi Paul,
With ANY wooden boat, when ashore and left in the boat park, open the hatch covers and the tank drain points, get out the water and allow the boat to "breathe". Wood takes up water. Give it a chance to dry out. Leaving damp to fester in a boat guarantees rot.
Fitting transom flaps is easy. Get some flexible plastic (talk to your local chandler), do NOT use perspex as this will shatter in next to no time. Drill a couple of 4mm holes in the top edge of the piece of plastic which covers the transom draining hole with an overlap of 8 - 10 mm all round. Screw the flap to the transom with a couple of 3 mm st. st. self tappers, but do not tighten them down onto the plactic. Leave the head about 5 mm proud. If the flap opens to let water out, it can move on the screw thread and will work well. To hold it in place, use a piece of shock cord (about 3mm thick). This is attached through a small hole in the centre of the flap (knotted on the outside) and goes to the other flap similarly. Put a small hook (speak to your chandler) on the boat floor/hogg and loop the shock cord over it. This will hold the flaps closed. The shock cord should be 'JUST' holding the flaps closed. Too much tension and the flaps will distort. In a bad capsize, reach back and unhook the shock cord. Refasten it when the boat has drained sufficiently. Also, if small bailers fitted, use MAXI bailers.
Mike.