prior to building the box just take general measurements of the area that you plan on building your enclosure to get a general idea of the airspace you have to work with. This way you can get an idea of what you are tuning your system for.
I have done a few fiberglass installs. Here is one back from 1995 that I did in an Eagle Talon:
Eagle Talon Install Photos by RDSAUDIOVISUAL | Photobucket