I made a tombola for the local school a few years ago. It was hexagonal, but the same principles would apply. As per suggestions you've already got, I used biscuits to reinforce the joints (getting the angles right was a devil, since I used a router not a b/j) and a couple of luggage straps from Halfords as substitute band clamps. It worked surprisingly well, and the tombola still gets rolled out every year. The glue stains are still on the front of my workshop coat though - although it saved a new jumper from a guaranteed ruining! Trying to get 6 sides all glued up, with the end pieces and then clamped before the glue went off was a real challenge :?
Cheers,
AG