forget the jigsaw...
watch
ebay for a couple days and grab a high quality used router for 50 bucks shipped... you can make a circle jig with a regular *** piece of wood and... theres your box //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif
personally, i'd decide on a height for the box,build the bottom with fiberglass, cut a piece of wood to fit the contour of the bottom of the box across the front (think template + router or jigsaw + sanding), and another for the back, then make sides and attach a flat top piece...
i'm also a fan of shimming the seats for extra mounting depth...