yeah, material is important, the 12" servo subs will load even an open baffle cabinet quite signifigantly. We use 1.5" MDF for the side panels, you could laminate 2 .75" sheets to achieve this if you can't source 1.5" ( I kind of doubt it). We also use 1" baffles/braces ( I like to build them like brick you know what's !) although, .75" will do. Our non-modular subs also use 1.5" tops/bases while the modular design use 1.25" side panels, tops and bases , 1" baffles, and .625" module uppper/lower plates which bolt to the next module l .625" bottom plate.
The material used in our modular design is Medex which is slightly denser than reg MDF so we felt we'd get away with the 1.25 in lieu of 1.5" ( can't get the 1.5" in Medex).
Even at that, it's still recommended to line the cabinets with no-rez