Before I bought a band clamp, I had a mitre section rebated on the inside to hold a ply base for a shelf inside my computer desk. To hold it while glueing I used some 1/2" elastic (look inside the good ladies sewing box) tied so it went round the section with enough pressure & used 4 L shaped pieces in the corners to hold it square. Its a cheap cheerful solution that works.
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 :?
Thanks for all the tips. I think as I'm going to be making a few same-sized boxes I'll have a go at using my metal bender to make up some 'clamps' out of 12mm x 3mm flat steel bar lined with self-adhesive door insulation foam, to protect the workpiece, and a bolt and wingnut to tighten up the clamp. If successful I'll post a pic
Don't hold your breath, I can't say exactly when I'll be making these things as I seem to have a permanently attached paint brush in my hand as my wife keeps me 'occupied' doing winter decorating :roll: but as soon as I can escape into the depths of my workshop I'll have a go at making these clamps and diy toggle and hold downs too :wink: