If you have had both fiberglass and wooden boxes in the past, this will help alot. I need 4-5 cubic feet of box space for my system, so a do it yourself wood might not work too well. What will sound better???
Fiberglass is an excellent material to build enclosures with. As a general rule, any thickness of fiberglass is as strong as twice the wood thickness. You can build a very strong enclosure with fiberglass and save a lot of weight as well. Constuction integrity is very important for subwoofer enclosures.
Due to the nature of the construction of a fiberglass enclosure (multiple layers of matt bonded with resin) makes the enclosure very neutral.
Fiberglass is a complex material to work with. If you have never worked with it before, I don't recommend this be your first project.....I'd stick with wood.
To build a subwoofer enclosure the size you're talking about, it would take much more time with fiberglass than wood.
Wood is good in a practical sense, but it doesn't allow you to extract all the area in a given space (fiberglass can be molded to irregular shapes).
If you are going to go through the trouble of building a fiberglass enclosure, don't cheese out and fiberglass it to the car. Make a plug than a mold than the enclosure and bolt it in. This will allow you to water-test the enclosure to determine how much airspace you have. The proper enclosure volume is very important to subwoofer performance. This will take a long time!
Shockingly, there are a few manufacturer's demo vehicles out there built by their installers who have no idea how big the enclosure is....but it looks cool.
If you're building your system under the shade tree, fiberglass is not practical!