After much searching I found some brass 1/2" NPT drain plug plates wihch are used on smaller boats for the stern drain plug. There are nice in that they have three screw holes and the 1/2" NPT thread actually extends out to a 3/4" protrusion which provides a nice tight fit for a 3/4" hole. (I failed to take a picture of the part but here is a link to the part I used. It's from west marine:
Garboard Drain Plug

I used stainless steel screws with large washers and I used double threadlocker stainless steel nuts on the inside. It was easy to reach through the inlet hole to tighten the bolts. I sealed everything with generous amounts of the slow dry 3M 5200 supplied in the kit.