Fiberglass is also alot more work were you talking whole enclosure but nikuk also right that is very true. fiberglass looks good but i'll go with a box anyday ported , vented sealed you name it fiberglass is nice but only if your going to show off that part of your car and you have the money also.
What do you think about those boxes. The specs are on the site http://www.4080enclosures.com and it is for the G35 coupe. I would get the passenger and drivers side.