Insulation for riser: what to use

[HDJK]

Hello,

I read a lot about the basics (38% into the room, as many bass traps for small rooms as possible, treat first reflection points,...).

For my home studio/home cinema I'm building a riser out of three of these: [IMG=http://img260.imageshack.us/img260/9999/eurpalette.jpg][/IMG] [IMG=http://img260.imageshack.us/img260/eurpalette.jpg/1/w120.png][/IMG]

As I've learned from the kind people of GIK, I won't be getting any benefits regarding absorption with this, but I need to fill it to avoid ringing. Now, what do I need to use? The same as for an absorption panel (Rockwool, rigid fiberglass) or is there something else that would prevent the boxes from resonating?

Thank you for any tip

Jan