I'll be using Parts Express #260-516 eggcrate foam on all the interior surfaces plus a layer of 3 1/2" fiberglass builders insulation on the back panel. It seems curious to me that an extended mdf panel would have no bracing - I would ask the folks at Zalytron whether they belive some interior bracing would be appropriate. It could be they omitted it to simplify the design, or maybe they tried an enclosure with bracing and found it didn't make a significant difference in the sound.