I am in the process of designing a subwoofer box very similiar to the one that Sound Illusions did for Big Ed a couple years ago. My question is for you box builders is: How are you guys going about sealing the plexi surface to the MDF?
Mask off plexi 3/4" from edge apply silicone to edge of box and install plexi. After plexi is installed reach in through cutouts for subs and remove tape. Remember it takes a fair amount of clear silicone because the end grain of the MDF will absorb some.
Single sided closed cell foam tape same width as MDF contact area and 1/8" thick. Silicone can make the plexi difficult to remove if you have the plexi as and insert to a face instead of being the entire face like in the picture.
Like Ben, I use closed cell weather stripping -- 3/4" thick. Sticky on one side. The pressure of the screws keeps the seal tight. It's easy to take the plexi off if you need to (handy so you can get the dust out from inside!).