It's been done before, 100% plexiglas box...
Maybe I should scan the pic of blaupunkt's demo porsche, it had a plexi bandpass..(hey, it was '96, EVERYONE thought bandpasses were awesome)
Even if you know someone who can get cheap plexi, it will STILL be expensive for you to gain 3/4" or thicker acrylic cheap. Drilling wouldn't be THAT hard, you could use wood bits and go slow, maybe a bit of lubrication to keep heat minimal.
You don't need to bolt it, if you are fantastic with acrylic glue. Acrylic glue would also seal the chambers really well, cause all it's doing is melting the plastic..sorta like wood glue. Silicone will look horrible i there, even if you use clear types.
Even if you get yourself a sealed enclosure, you better make sure the cuts are done with pin-point accuracy. It's gonna be a pain if you don't apply the right amount of glue on it. one smear and your box is ruined.
You also gotta know that plexi is sooooooo easy to scratch, easier than CD's. I guess you gotta cover them with something when on the move.
Sony C90, 210 EQ
Seas 27TDF/C, RE XXX 6.5, custom xo (dyn 240GT sitting idle)
RF 60.2 amp
Soundstream picasso 1400.1D
Bass drive-by's are uncool.