I had a vintage plastic full spout and I laid the cap up in a cubby hole where, I always thought, it was safe until my 12 YO nephew came aboard and kicked it over board. It turned out to be a standard 2" straight pipe thread. So I bought a plastic 2" pipe plug
and machined it down with an O-ring grove to fit the hole. And milled a slot w/end holes for wrenches.
This cap I put on a 1' lanyard with a clip on the end so that it fits the safety
lines, since there wasn't a place to attach it inside the fill spout. I just have to be careful not to drop it.