. . . the rez fills up with roots. Its a lot of work to dump out and clean. Any idea on keeping them out?
You might try adding a layer of cloth between the netpot and the soil above it. If you use something like the stuff airpots are made from, so water can pass through, but soil and roots stay put, that would make cleanup much easier.
I doubt the layer of fabric would interfere with wicking between the rockwool in the netpot and the soil above.