Basically, all you do it take the old stock pod and make a copy out of it using 1/2" or thicker wood for the front piece,
and 1/4" or 1/8" masonite for the back piece.  then just stick some chunks of wood as spacers to hold the front/back
 pieces the correct distance apart.  cut a hole to fit the speaker on the front, and cut out as big a hole as you can 
(inside the spacers) on the back..  seal around the spacers and stuff with fiberglass, bondo, or whatever you can use
 (fiberglass works the best)..

then let it dry, screw a speaker onto it, and install in the car.

that's about it.