Do you have a closer look at the hitch?
I think you could :
– use a piece of air hose (or hydraulic hose) on the hitch to "wrap around" the attachment ;
– use a nylon strap (or something like that) as a "safety harness": it would run from the trailer, around the seatpost and back onto the trailer, where you would pin it in place.
Maybe there is just a question of angle. From what I see, you should be able to bolt the post (at the front end of the trailer) many centimetres lower so the trailer would be straight. This would certainly make the loading easier, and maybe the hitch will be at a more stable angle.