I just finished my first DIY speaker project (and my first time using some tools to make anything). Its a lab 12 in a 20" cube, I believe tuned somewhere around 25hz. Inside is a cross-brace and the walls are lined with acoustic foam.
I want to say thanks for recommendations on truck bed liner. I used dupli-color stuff from wal-mart. The coarse roller I used was quite absorbent and a bit of a pain in the ass to use. 1 quart lasted for 1 coat, not including the bottom which is still bare. I think for my next project (might be a keyboard cabinet for my friend or a cab for 2 10" subs in my car) I'll try it with a less fluffy roller and see how it turns out.