The nothing but net balls looks suspiciously like the small clean sweep balls and the balls in the classroom competition. (they are all 4 in in diameter)
Do you think I could use old clean sweep balls/ classroom competition balls in place of the nothing but net ones until my club gets a field if I want to run some tests with different wheel types?
I can’t see a problem with using the clean sweep balls for testing. It will at least let you check your designs. Hopefully if there is any difference in the balls it should require only minor tweaking. The only problem that could occur is if for some reason they use a different ball material / density which is unlikely.