Thanks for the advice. I was planning both the enclosure and the dynamat, or at least material like dynamat.
Planning this out further it looks like the best place to place the sub is on the driver's side of the trunk, against the inside of the rear fender.
I'm using a rubber paint on as much of the metal in the car as I can in order to eliminate rattles. This includes the rear of the dash, the center console, brushed aluminum panels, roof (under the headliners, floor pan, door skins, trunk sides and floor, etc.
After that I have some 3M automotive product to use on the driveshaft tunnel and sides of the floor pan.
I've got a new firewall pad on order to replace the original stinky one.
My body guy tells me to use a product available at Lowes for additional sound deadening. It's called ecofoil, and is about 1/4" thick and has a center layer of basically bubble wrap, coated on both sides with foil. It's made to insulate attic rafters to reflect heat and to absorb sound. He says it's just as good as dynamat and is a lot less expensive. I'll insulate the cowl, roof, floor and line the truck with that as well.
2005 4.0 MT dual exhaust GT mufflers GT suspension and wheels