We came up with a pretty simple plug for the "blowhole" effect in the M17 by slicing a piece of 2 inch long rubber tubing (something like the black rubber automotive fuel line tubing) which had an ID that was a tight fit around the line used to draw up the keel and had an OD that was a snug fit into the "blowhole". Actually, I trimmed a section of a piece of tubing that was too large to fit in the hole down a bit so that it would not get permanently stuck down in the hole. So all in all, it makes for a nice gasket to keep the water out of the cockpit. Having said all that, we still get water in the cockpit because even in a mild chop water seems to flow in through the stern cockpit drain holes. They have backflow prevention devices inline, but for some reason a trickle will still flow through and after awhile there is around a half inch of water sloshing around in the aft section of the cockpit. It does get above this amount because then it starts draining back out. I was considering installing some sort of ball valve shutoff for the two drain lines, but it would be bad to forget to open them up and then get a serious amount of water suddenly in the cockpit. :-( Maybe the solution is to always wear heavy rubber boots...