Thanks! The hitch wasn't perfect, but it worked. It was actually meant to be temporary. After finishing the trailer, I spent quite a bit of time looking online at various instructables to figure out how other people had attached their trailers, with the plan of switching to a better one. I never did; the tubing worked well enough. It did finally break after a few years (and it was clear that it was breaking, as it had stretched quite a bit). I had more of the tubing, so I just made another loop.
The black plastic part that's attached to the aluminum rod has a hole in it. I don't remember what it originally was on jogging stroller, but it had a metal pin that went through it, with a gap in the center. So, that part was already attached and made a perfect entry point for a bolt (I later replaced the bolt with a quick-release skewer, but the picture above shows the original bolt). I have a small length of plumbing tubing, which google tells me is officially called "vinyl braided tubing". Really strong stuff. I flattened it out as best I could and drilled holes in both ends of the tubing. This allowed me to make a loop with the tubing, with the bolt going through both holes of the tubing and the black plastic part. For my bike (which had a pretty heavy-duty rack*), the best place to attach it was the end of the rack. If the loop is too big, the trailer won't attach very well, so the loop should be just large enough to fit around whatever you're putting it around.
If the rack wasn't there, I probably could've used the seat post. Even with the loop being tight, there's still a bit of give as you accelerate and brake. The trailer wants to stay in motion. One way I screwed up was by not putting any rubber around the end of the aluminum rod. It rubbed against the rack quite a bit, scraping off paint and bits of metal over time. Not a huge deal for my rack, but I'd have been pretty annoyed if I'd attached it to the bike frame and that happened.
* Axiom Journey racks are these sweet little $40 racks that can accept 150 pounds (70kg) of weight. I can't recommend them highly enough. There are multiple rack manufacturers that use the name "Axiom", though, so make sure you're getting the right one.
