I still calculate enclosure volumes when working with fiberglass to within 10%. Sometimes the subs can be in a flat box with a fiberglass trim or cover like in the Mercedes posted here a while back that I built.

For SQ competitions cars its trial and error. When doing exotic fiberglass enclosures, sometimes the angles or any other factors change the sound. In my high profile competition cars, Ive built several enclosures over and over for each vehicle. Then we do RTA and LEAP analysis and measurements inside the vehicle. We also run test tones to find resonant frequencies of the vehicle for SPL competition vehicles. Analyze the data from different enclosures and pick the best or keep building and experimenting until we get as close to perfection as time will allow.