I had to make up a frame with side pieces thinner than the top and bottom so mitres not at 45 degrees. I did my best with a hand mitre saw but wasn't too confident about how square everything would end up so it occurred to me that I could fix all the sides, face down to a cutting mat, using double sided tape. This way I could be sure it would all end up the right size (handy grid on the mat) and square when I glued up to the backboard. I used PVA with dots of CA to hold everything together. There may well be some happy joints but I can deal with those (it is all going to be painted) and hopefully it will end up flat, square and the right size!