3D Printing Rocks
I decided that, rather than do yet more simulation, I'd start trying to prototype my gates.
The simplest way to do this seemed to be designing them in some CAD tool and having a 3D print done. I spent a number of weekends going through Blender tutorials, only to figure out that it just wasn't the right tool for the job. It's much more aimed at making cool pictures and animations, and not so good for constraints like "all walls must be at least 2mm thick". After getting stumped there and working on other things for a while, some friends turned me on to OpenSCAD. I'd just like to give a big shout out to Clifford Wolf, the author--it's a great tool, and I had my first design in about 1 day of work. I tweaked it a bit more before I had the prints done, but still, it was really fast to work with.
Here are a couple of shots of the gate I printed; I got one side in transparent plastic so that I could see what was going on inside, but I got the rest in white because it was much cheaper. Big thanks to shapeways.com for getting me my parts in 9 days, upload-to-delivery, for half the price that I could have gotten it done in the U.S.
Here's the fully-assembled gate, lacking only the metal axle, a 3/8" D-profile steel shaft. The shaft's way bigger and heavier than I wanted, but apparently it's hard to find small, light shafts with that profile.
Here are the 3 parts laid out so that you can see inside. Those circles and doughnuts on the transparent gate are actually on the far side; they're alignment pegs and sockets, and the white one has them too. They're so that you can slap a whole stack of these gates together without worrying about binding on the axle.