Yes, you MUST use fiberglass mat. I haven't done any fiberglass either, but I plan to (attempt to) build an enclosure for the side of my cargo area.
Here are several online tutorials that I've been studying. Of course each is basically showing the same process, but some things are shown/explained better in different tutorials.
Step By Step Fiberglass
Building a Fiberglass Speaker Box
Building my own custom sub enclosure for cubby-holes - Scionlife.com
Fiberglass subwoofer box - a tutorial
Here are several online tutorials that I've been studying. Of course each is basically showing the same process, but some things are shown/explained better in different tutorials.
Step By Step Fiberglass
Building a Fiberglass Speaker Box
Building my own custom sub enclosure for cubby-holes - Scionlife.com
Fiberglass subwoofer box - a tutorial