MegaHitch Lock Breakaway Security Nuts are a great alternative to coupler is not welded to the frame of the trailer. The cone shaped nut replaces your existing nuts that secure your coupler on.
The nut and bolt size is 1/2".
When torqued down the hex head shears off leaving only the cone shaped head. This makes it impossible for someone to place any kind of tool on the head and remove the nuts. Nuts can be purchased in a set of two.