I used a minimalistic amount of dynamat and fatmat... whatever was on hand and cheap!
My fronts get enough punch for most of the rock/metal I listen to. Sure, a sub would be nice... maybe in the future.
For light weight, there are a few options...
heavily braced 1/2" MDF
Fiberglassing non-flat boxes (spherical if possible)
Infinite Baffle
I'll be going the infinite baffle route, if I end up with a sub. I'd prefer a single 10" or larger... but depends on the subs of course. I have 2 18" Fi Audio IB subs in my home theater... plays flat to 7hz.

lol