yea dude you should prolly cut as much as you can on somthing like a ban saw with a straight-edge-jig [similar to a tablesaw.
but you can buy plexi blades for a tablesaw, just make sure you dont take the protective coating off the plexi until you are ready to actually USE the box.
this is also the glue you will need;
http://www.misterplexi.com/solvent.html
you should buy a bottle with a syringe-type-tip because the stuff is real thin - DO NOT SPILL IT ONTO PLEXI! it CHEMICALLY 'solvates' the acrylic - ie melting it - and then when the stuff dissolves away the plexiglass returns clear and hard again, but the edges that were liquid turn to a solid but are now "one piece" and its extremly hard to break that bond.
you will definatly need a really fine-tooth-blade for this - and put some duct-tape over the edge you are cutting, it hlelps not-chip the plexi [wich is what you do not want, so be sure to buy a brand new blade when you cut it on a tablesaw]
you can then clean the edges with a sanding block [a long one] with some 120 grit wetsanding paper and then use a heatgun to "melt" the smoked edges to clear again... just make sure your cuts are perfect because plexi is not forgiving like MDF is.
AND FOR CHRIST SAKE! PRE-DRILL YOUR SCREW HOLES and cut the speaker hole BEFORE you attach it to the box - and cut it with a scrollsaw or somthing. DO NOT use a jigsaw. but use T-NUTS AND BOLTS for the sub!