WTF? No no, you've got it all wrong.
First of all, don't ever use wood for your enclosure. Unless you pay for high-end exotic wood the best thing for you to use is Medium Density Fiberboard, or as it is commonly known MDF. Don't use wood, don't use plywood, don't use plain-old fiberboard...use MDF.
Now, MDF has pretty much the best acoustical properties you can ask for. It is very dead and will absorb frequencies without coloring the tone of the bass. It doesn't create a perfectly sealed box since MDF can breath a little , but if you build the box right it shouldn't affect the sound.
Fiberglass, no matter how thick you make it, will not be as dead as MDF. It will still vibrate a little bit and add a color to the tone of the bass. I can't describe the sound other than to say it isn't quite as deep. However, a fiberglass enclosure will be perfectly sealed if done right and no air can pass through the walls.
The only advantage fiberglass has over MDF is that it allows you to utilize space a little more efficiently. For example you can get the maximum airspace out of a tire well or the corner of your trunk. With MDF, you are usually restricted to rectangular or wedge shapes. If your carpentry skills are good you can do a little more with MDF but usually people use fiberglass to do weird shapes and smooth curved panels.
Now a bass breather...I assume you mean a port. Designing a subwoofer enclosure with a port is not as simple as sticking a hole in the enclosure and shoving a pipe in it. You must take into account the specifications of the subwoofer you are using and design a box appropriately. The volume of the box, the diameter of the port, and the length of the port must be precise.
Whether you go sealed or ported, MDF or fiberglass, check your subwoofer manufacturer's website for suggested box sizes, dimensions, and port lengths. If they don't have one that suits your needs you will have to download and learn how to use box building software. There are several programs out there (the good ones cost money) so I'm not going to list them.
Hopefully this gets you pointed in the right direction.
