I have a Guardian model G44 trailer coupler lock for sale. It's made specifically for UFP couplers. I used this on my old LSV when it sat out for the night, which was maybe 4 nights a season, so it's in like-new condition. Comes with both keys, box and instructions. $25 + shipping.





There's additional info here on this model:

http://www.guardianhitch.com/g44.htm