A generic twistlock NEMA L5-30 receptacle, plug or connector body for the cord can be found at most good building supply and hardware stores. Finding a water tight version usually means going to a good electrical supply store or using the net.
Marinco makes marine NEMA L5-30 receptacles and the result is a somewhat water proof connection when used with a standard connector body. It just requires more pieces to setup a receptacle. My sailboat connection was corrided after a couple of years in a salt water environment, so they aren't the best.
We used lots of Hubbell twistlock, watertight connectors at work, up to 480 VAC three phase, thus my preference for them since they functioned so well in a wet environment. They can be found at
Newark Electric,
Allied Electric or other suppliers on the Net if you do a search.
btggraphix - I took your advice on my original post and sealed the edges of the insulation. You should still make the change when it gets warmer ;-)