I won't be making a fiberglass enclosure.
I will use the spare wheel itself as the enclosure. Holes are already sealed with a big rubber seal and I have machined a big(6") "washer" which is mounted over the rubber and bolted to the car using a regular head bolt. Gives me more room for the speaker magnet. If I remember well there is around 24-25L in there. I have filled it up with water a couple of months ago.
I will place a MDF board, over the spare wheel and install the sub in it. The only two things I haven't figure out yet, is how to bolt down the MFD against the spare tire, to seal properly and how it will fit under the factory floor.
I may drill and tap, like four holes into the spare and use threaded rods.
This would be my first sub/amp install. Polk Audio PA660/PA880 amps repaired by me. Picked them up from the bay for $40 each. With the JL 10TW3 shallow sub, I need a minimum of 14.16 L(0.5 cu/ft) space.
http://www.jlaudio.com/10tw3-d4-car-audio-tw3-subwoofer-drivers-92184
Right now, I have two pairs of Infinity components, plus the Polk Audios in the dash and I have a very good bass, so two 10" it would be to much in my opinion.