Sonex pyramid
http://www.markertek.com/MTStore/product.CFM?BaseItem=PYR-2

standard foam
http://www.foamorder.com/products.cgi?cart=i3X0q0n822M622M53M66n4B1b0Y&item=acoustic

configuration;
* Lambda TD15 woofer
* 5ft^3 rear ported box tuned to 35hz
* 1.5" thick MDF cabinet all around.
* crossover - low pass 800hz - hehe
* 500w feeding the woofer.

Should I increasing cabinet thickness
to 2.25" instead ? hehe

I'm the uber-overkill type of person
so feel free to post esoteric sound
damping ideas - lol

You can get barium loaded vinyl in sheets which is a sound deadening material. it's used in sound proof rooms. idealy you would make a sandwich with MDF|vinyl|5/8 drywal or mason board|vynyl|MDF
other alternatives could be linolium, modified asphalt roofing materials, sand, lead sheet, liquid filled core. The entire box should not be one box. the layers can not be bonded together. you would in a sense have a box inside a box, inside a box. a lot of unequal and random bracing would help too.

good luck