I'd hate to search police you, but if you search this forum you can find many ideas, rather than let this post be just that, here's some helpful info:
It depends on how you want the finished product to look.
If you want a square (rectangle) box (easiest) just use MDF (at least 1/2" but 3/4" is more recommended.) You can pick up 4' x 8' sheets for about 20-25 bucks at home depot/lowes. Just make sure that you build it to the right interior volume (don't forget to subtract the volume of the sub itself.) it's on your sub's info sheet, or rockford's website, or as a rule of thumb per sub: (for sealed chamber boxes):
8" - .66 - .9 cuft,
10" - .75 - 1 cuft,
12" - .9 - 1.25 cuft,
15" - 1.1 - 1.5 cuft.
(Note that the larger the box, the "boomier" it will sound, the smaller the box, the "punchier" it will sound.)
The best is to use a separate chamber design so that the subs aren't "fighting" for air-space or against each other. (This applies with the following designs as well.)
A flat box, but with angled edges (to match angle of seat back):
Use the same method as above, but cut the edges to angles.
A combo of flat and round pieces:
Use MDF for the flat parts, since Fiberglass isn't strong unless it's curved. And make a sort of skeleton for the fiberglass parts (mdf is again easy). Lay fleece over and stretch and staple. Apply resin. Repeat a few times until the thickness is about 1/2" or about 5-7 layers of glass (on top of the fleece) are down. Sand. Sand. Sand. Sand. Filler. More sanding! If you are carpeting, you would need it to be fairly smooth, if you are painting it, then it needs to be PERFECTLY smooth, otherwise imperfections will show once the paint is done. (This forum has a LOT of threads about how to build FG boxes, also Alpine uses alot of FG in their demo cars, spend a few minutes there to get an idea of what they've done.)
Odd-shaped, custom molded, or wheel well designs:
If you are looking to install these behind the wheel wells, you will use a minimal amount of MDF, and maximum amount of FG. Lay down a release agent (most people will put down blue masking tape covered with aluminum foil on the carpet to keep the shape, and provide an easy release. some even spray cooking oil (pam) on this to aid in release when everything is cured.) then apply the first layer of glass directly on the carpet/foil. apply at least two more layers before removing from the car to add more layers (to about 5-7 total as above). Then follow the above steps, but add the glass to the inside of the box, to keep the back shape. For the other "half" of the enclosure, make a sub ring (usually out of MDF) and "hover" it where you want the sub to be in the end (and anything else, amps, etc.) Use hot glue and dowels or more MDF to get it to stay (make sure it's secure) and lay more fleece over this connecting it to the back half. Lay down more glass, and begin the finishing process (sand, sand, filler, sand, filler, etc.) Make sure that the boxes are symmetrical in volume for the subs to sound right. (within 5% volume difference.)
keep in mind that non square shapes are hard to calculate volume. The easy way is to get a cardboard box that has the right volume, and fill it with packaging peanuts, then pour them into the completed box, as long as it's pretty close, you should be just fine.
Also, make sure to seal ALL edges, for MDF, usually foam insulation or silicone sealer (in the squeeze tubes) is best. For fiberglass, a layer of glass and resin on the inside edge works really well, but is sometimes hard to access.
In any case, keep in mind that you will need to run your speaker wire to the sub, and it would be important to seal this hole as well.
In the end, the best for your set up would be behind the wheel wells. It will allow you to fold down your seats, and still fit big things in the back.
I would use the edge of the load floor as the maximum distance the sub box should extend into the trunk. (where the gloves and rope lie) I'd also try to follow the curvature of the wheel wells, so that it looks like it's supposed to be there. Mask the floor past this line, glass up to and slightly past this line, you can always remove material later. Then when the back piece was done, I'd make a template of the wheel well curve, and cut out mdf to make "ribs" to attach the fleece to and glue them to the back half. On the right side, there is an indentation, I would put the amp on this side, and fill the space so that it is the same volume as the other side.
Most people would flame you for not running a quick but you got lucky this time.
Have fun with your project!