Lewmar
I just installed a Lewmar Ocean 60 hatch on my '83 H25. It has an out-turned (flat) flange that bolted directly onto the opening without ANY modifications. I just squeezed a bead of 4200 around the opening and set the hatch loosely for the night. The next day I tightened down the screws, and voila!, no leaks. No muss, no fuss, no hacking out the opening, and I have a hatch that will last a lifetime, not a plastic cheapie that will cause more problems in the future. It was pricey ($550), but worth it to me just to keep from chopping up my boat.