That hitch design doesn't allow much vertical movement of the trailer relative to the bike. Why don't you orientate the rod ends the other way, like this....

These plates have a nut welded onto the insides, you can just make out the thread...


The rod ends are attacjed to the swingarm by threadind them onto high tensile bolts welded directly to the swingarm. This allows you to 'tune' the length of each leg of the swigarm.


I welded wingnuts to the bolt heads that attacj the trailer to the bike. Makes it quick & easy to hitch the trailer to the bike.



The swingarm is a steel fork with the legs cut off and welded at 90 degrees.
The finished product...