Get some neoprene collars that fit the net pots. They should have a split in them, that is where you pt the germinated seed or clone. Using rockwool in aeroponics is not a good idea because the rockwool will hold too much water. I don't use any medium, just spray the roots. Post some pics of your aero unit so we can see what you are workin with. Lots of experience on this site to get tips from.