When I had my Pup, my brother took some square tube and measured the wheel hubs and made me a lock for the wheel. It was like an "L" shape but with a shorter tab on the bottom foot. The shorter part with the tab fit around one of the "spokes" of my tire from the back. The longer part of the "L" came through the "spoke" area to the front (barely) where he had welded on a bar lock he found at Granger.
The only way thieves could have removed it was with a cutting torch. I let it go when I sold the Pup since it probably wouldn't have fit another trailer.
I have HEARD that just putting a locking ball in the coupler and/or chaining it up won't work because thieves will just wrap a chain around the tongue and tow it a couple of blocks. Then they bring a welding torch, cut off the old tongue/coupler/chains, weld on their OWN coupler, hook on & drive off.