I like to use the HZ Super Cub for the same reason you like the SPAD. Tough, cheap, and easy to repair. Before I started using a buddy box, I've had students crash them nose first with fuse and wing both broke completely in half and I glued and taped it back together and flying again in 15 minutes.

Now that I also use buddy boxes and nitro planes, there's another trick that seems to work on a lot of people. I give them their first flight on the buddy box with the Avistar, then we go back and start over with the Super Cub. For some reason, after they struggle a few minutes keeping up with a faster plane, they do much better when they get on the much slower Super Cub. I also usually solo a student with the Super Cub first, then the Avistar. ! Kudos to you for giving unselfishly of your time, keep up the good work!