I'm a big fan of packaging tape to at least hold the pieces together while you clamp it. Do a search for " miter fold" and you'll see it. Basically, lay the pieces side by side with long points touching and facing up. Lay down a strip of tape spanning each piece, you'll then see that you can fold them together and tape will keep them aligned. Turn them over, apply glue and fold them together. Then add some clamps, tubing or just wraps of tape. Also, those offcuts from when you ripped those pieces can be used on the outside to make 2 parallel edges for clamping, hot glue them in place, or maybe packaging tape will suffice there as well.
Whatever you do, be sure to practice before glueing.. Good luck