I used ‘hardware cloth’ for the ambient plots. It comes in various gauges and you can get it at pretty much any hardware store. I used it primarily to keep the rabbits out.
Yes, the hoops served as a guide for planting. The kids just had to make sure that all the plants were inside the hoop.
I didn’t want the plants to be located too close to the sides of OTCs, so all of the hoops are the same size as the top opening of the cone OTCs.