Brad, you mean actual
hatch boards? Or a
hatch, a sliding or hinged
hatch itself?
If you're trying to replace rotted wood for either, a good way to do it from scratch is to cut large flats of corrugated cardboard from appliance boxes (or buy foam board from art supply stores) and make up a dummy from that, since it is easy to work with. Once the dummy is sized to your satisfaction, you've got a full-sized template to use for cutting the wood and other materials.