Hi Noel,
This is not the cheapest trick, but it's what I use. I bought a spare 'trailer' pigtail from the dealer and cut off the extra wire (better to keep the wire and simply tie it neatly to the side in case you ever need to use the plug). Naturally it plugs onto the truck pigtail abd provides a perfect seal. Quick and easy. If you want to really go crazy, buy a spare 'truck' pigtail and use it as a plug for the trailer. See ya,
Joe K