I am going to opt to put them in three gallon pots filled with hydroton, I agree it cuts down on price, but more importantly it gives you some mobility within your tray. I am not fully understanding what you mean about blanks displacing volume, but we aren't all together super concerned about the weight of the 4X8 because we built a sturdy structure to support the table and also have supports in the middle of the table so the weight should not be a big issue. Using the braces under the table should help to absorb the weight of the table in three sections, so I don't see a huge problem.

Trying to stay away from rockwool, pretty sold on hydroton, have seen good results using that as a medium. Love the fact you can just wash em and use them over again as well! =)