Tom,
You should build the bottom portion of the box with fiberglass and MDF. Just putting some wood at stock floor height will probablly not give you the necessary internal space. You way have to raise the floor a bit to accomodate the ol' MacDaddys. (See Jeff M's box? We built that and it has been raised about 2 inches to give it more room.)I can just say that you'll need to internally brace the hell out of the box if you are planning on putting a serious amount of power to the subs. I have my box built with about 1 inch of glass and until I internally braced the box, could hear it flex.
Robert,
I have my entire car sound dampend with Cascade V-Block. It is similar construction as the Dynamat but I feel it is easier to work. Nevertheless, I have done my entire hatch area and the rear lift up hatch with the vinyl matting they offer and it makes a huge difference. Once you remove the spare tire and all the other stock foam parts, tools, the road noise is terrible! It is very much recommended that you use some sound deadening becuase the road noise will be way too much to overcome. (It bugged the shit out of me for the first week I had to fix it!) You will be able to purchase enough to do the ENTIRE rear of the Supra for about $120. It is well worth the money regardless of the brand that you choose.
Tom Kenyon
Custom Sounds
San Antonio, Texas
210-340-4000