Fiberglass is pretty strong and people have used it to good effect. I think that in a sub application using fiberglass, just try to make it as thick as realistically possible. The shape of the box really doesn't matter, it's all about the volume of the enclosure. The volume of the enclosure will affect sound quality and power handling. Each subwoofer has its own characteristics that you can use to design an enclosure that will hopefully meet or exceed your tastes. To figure out the volume of a fiberglass enclosure, can't you fill it with water after it's cured?
ported, sealed and bandpass enclosures differ very greatly with respect to sound quality and output levels. Ported = loud, and if you design it right, can go very low and have sound quality, but this depends on the subwoofer driver too. Bandpass = loud, but in a certain frequency range usually and doesn't have very flat response and tend to not have good sound quality unless you know what you're doing. I personally have no clue how to design a proper bandpass enclosure. And, sealed can arguably produce the flattest frequency response and best sound quality, but usually at a cost of loudness.
Yes, you could build a fiberglass enclosure of any shape and stick a sub in it. And about the wood, I heard Marine Grade Plywood (9ply or greater) has very good strength and will outperform 1" MDF. Now, I can't exactly explain this, I just read some audio heavyweights, such as dan wiggins (of adire audio) say that he prefers both marine grade plywood and baltic birch over MDF because they are stronger. The disadvantage to Marine Grade Plywood or Baltic birch (9-13 plys) is that it is much more expensive than MDF. MDF does work great and 3/4" is good as long as you use sufficient bracing.
And, I have no idea how thick the fiberglass would be in order to acheive the same performance as 1" MDF.
DEQ-P9 (30band eq L/R, time alignment, 36dB/oct 4 way crossover)
Soundstream Tarantula (for subs)
Precision Power PCX-4125 (for horns and midbasses)
Image Dynamics CD2 comp mini horns
Scan-Speak 7" 18W/5535A midbasses
Subs still pending.