Anyway, put braces tying front to back panels together and side to side. The braces should be put between the woofer and tweeter. R13 insulation should be put on the back, bottom and side. Also hang a sheet on the braces, spanning the cross-section of the cabinet. This will attenuate midrange inside the cabinet but let bass pass right though. As for wood to build your cabinets, use MDF or Baltic Birch. The braces can be made of any hardwood, 1"x2" works great.