Stay away from Dynamat, it is extremely over priced. PM ifandorbuts to get some of his deadening material, I have heard good things about it.
As for the box Q, I don't think you will find anyone here that is a fan, or will reccomend for that matter, a pre-fab box. You can make one your self like the one you are looking at for about $25...now would be a good time to practice. I would make you one for about $40+s/h...sorry, I'm all out of those speaker covers. I know there are people on here that make boxes that are closer to you though so you don't get bent over for s/h charges. Post something in the box forum.
2006 Jeep Liberty
Adire Audio 15" Brahma running off Cadence A7hc, Clarion hu, MB Quart 6.5s and 5.25s running off old school Punch 800a4
146+db @ 38hz with 1200w and a single 15"
Lv = [(14630000 x R^2) / (Fb^2 x Vb)] - 1.463R
*Vb = vol. in cu"
**R = sq.rt. A / Pi (for square/slot port)