First off, how wide is your tray? Please give pictures, and also, use plates or even c-channels for custom screw joints. If your tray folds backward, then you can use vex hinge pieces. If not, then a screw joint on the side of the tray will suffice. If you have to, look up 448x’s easy design (the documentary) for inspiration to see the concept, but do not copy thier design, as making your own design is a good learning experience. Good luck on your tray. Use counter sink screws if you have a skinny tray. Ours is 9 holes wide and a tight fit so we had to attach our joint to c-channels which were below our tray.