steal works great with concrete behind it. no flex at all.
kinda hard to awnser your question with out knowing your setup. what are the demensions of the box and what kinda power and woofer?

 

More internal bracing is just as good as thicker MDF. Don't use anything thinner than .75".

 

Normal MDF will still breath a little when its untreated (ie: no paint or resin). Try putting a piece on top of another, then putting a good shopvac to the top one. Even at 3/4" you can usually still pick up the bottom piece. Doubling up the sheets will reduce this breathing, and depending on how much glue you use between the layers that may also be acting as a sealant.

 

interesting

 

I have double lined in my truck but then again my sub weights like 100 pounds.

 

and i was sure you'd be the one to know that size dosent matter[8D]

 

Mr. Loud where were you at the Sask SPL shows?