It will work, but you have to make sure it's VERY strong. Making pods for smaller speakers is one thing, but a large sub with lots of power is a different monster. No air bubbles, lots and lots of layers, and even rope in between the fibergalss. Yes, people use MDF because it's easier and MDF is better if glassing is not done correctly. Is there a reason you want to glass? You can shape your box with MDF and make a beauty panel. Heck even make a beauty panel out of glass if you want. Are you trying to fit to a certain shape in your car for stealth?