I posted some pictures here:
It did very well: we used up the full tether, and got most of the way across the pond. The kids had a blast.
We need some kind of basic stand to hold the spool ... the kids got tired of that job and it quickly became my job!
Also, one endcap of the electronics tube was ALMOST open when we finished; I think I must not have let the air warm up enough before releasing it with the plungers. I also think using the o-rings to hold the tube in place is dangerous: the o-ring wants to squeeze into that opening and pry the endcap off. Not sure yet how to make these more secure...