What medium are you growing in beside the chunks or is it an exclusive use as that as a grow medium.
I would fill 3" net cup and cover RW chunks in medium size garden stone.
The stones will protect the RW from light as well as hold it in place.
Holding it in place will give a firmer grip to your roots.
if you started them in net pots;
as the roots grow thru the chunks and out of net pot, you simply transplant into larger net pot with small pot inside the large one.
3" could go right into 6".
net pots work in every hydro system I have ever seen
(that is not saying much, I am a Noobie,lol)
from E n B to Aeroponics they all can use net pots.
I cover my RW with Hydroton, many people will hate on both products, use what works for you is my advice.
Bless,
DZ