For what it's worth... speaking as a sailor who loves to sail fast on a sports boat (with gaskets), your PMD does not need these gaskets (I had initially contemplated this as well).

In fact such a gasket would only serve to catch sand and dirt and what-not every time you pull it up on a nice beach for a fine lunch. Leave the slot open and clean. When rowing (and not sailing) you should cover the DB slot either from above or stuff some closed-cell foam inside the slot so that your underside does not get soaked from the wash in the slot.

I fashined a thin mahogany cover with a slot gasket that sinks into the slot and plugs up from the top.

Finish the boat and use it for a while. If you still feel that the gaskets would make sense, then go ahead and try them.

Chris

After many sailing years myself and later full keelers. I have come to the same conclusion that it would be more trouble than it is worth. Just a thought however.