Best way to decrease the internal volume of a sealed speaker?

I just installed the Trex 2x4's.
I installed (2) of them 18" long on each long side placed in the middle of the cabinet.
I figure with that wood my cabinets math out to be .62 cu feet. I already can tell the difference with tighter and deeper bass. The woofers will probably need some time to break in fully, but at this point I am very happy.

I really want to thank you all for the ideas and support. 

ozzy

Good to hear! (See what I did there?)

Synthetic wood was probably a good idea.. denser and not affected by humidity. Well done

Trex....yeah, better than that pesky cellular stuff....good call. *G*

The problem with those " quantum mechanical devices" is, if it 'goes south' (or Whatever 'direction' one wants to call That...) is that parts are just insane to procure....it's not the $, it's the shipping and the 'back order wait state'....

I’ll second georgehifi’s suggestion of liquid nail and bricks - with a twist; use 4 bricks. Glue them as close as possible to the centres of the sides and back and the available space near the tweeter on the front. Help strengthen and damp the walls while you reduce volume. I believe most cabinets will benefit substantially from bracing. Let you think about it. Cheers. Pete

Google what is trex made of. All the same crap mdf is made of except trex has the added incredient of shredded plastic and probably more ga -luue as a binder for this compost pile. Tom