Fitting 2 15s in a celica

Sounds like a great opportunity to use some fiberglass.

Have a look in the fiberglass forum for some good tips.

Mask off the area you're molding real good.  Wear some old clothes with long sleeves- it's no fun getting resin on your arms, especially if they're hairy.  Make sure you lay down at least 2-3 layers (maybe more depending on what weight mat or cloth you use) before removing the mold from the vehicle so that it doesn't warp and lose it's shape.  Also be sure you don't make it too big to remove from the vehicle.  The smell of resin can really get to you as well as the glass fibers floating around- wear protection accordingly.

Those are the main things I can think of.  Go to the paint section of the hardware store and pick up plenty of cheap bristle brushes, plastic mixing pails, latex gloves, acetone, fiberglass resin and mat/cloth, etc.  You may want to get some extra hardener for the resin.  Make sure the pails you get are the ones with markings on the side so you know how much resin you're using.  I usually mix resin in one pail, and keep some acetone in another to clean my brushes after lay up. 

If you get resin from the hardware store in gallon containers, it'll come with two tubes of hardener.  Each tube has 22ml with markings on the side to help you guesstimate how much to use.  A little basic math will come in handy.  The amount of hardener I use varies with the outside temperature.  At 70 degrees F, I use two tubes of hardener for every gallon of resin.  If it's a little cooler outside- 60 degrees or less, it'll take a little longer to cure, so I sometimes use a little extra hardener to speed this up.  If it's a little warmer- 80 or more, it'll cure faster, so I may use a little less.  This may not matter if you're working inside a heated garage- just make sure you have adequate ventilation.