You could use a steel spring like the ones in old bedframes or around trampolins; it should be possible to get one from the nearest scrapyard or recycling centre. My trailer has such a spring by design and it works just fine.
In my experience the forces on the coupling are not enormous. The largest forces happen when you brake, but these are compression forces and shouldn't tear it apart. The pulling forces are normally not that large.
I would also add a safety rope from the trailer to the bike. Fix it to the bolt on the right side of your photo, and tie the other end to the bike frame, with just enough slack so that it doesn't stretch when the trailer moves in all directions and doesn't get caught in the wheel. If the coupling fails for whatever reason, then your trailer doesn't get out of control.