Drill screws into the 10' board to a 6' board at the corners, using a 90 degree angle. Drill another 6' board to the other end of the 10' board, using the same 90 degree angle. You should have a rectangle shape with only three sides. Repeat these steps to make another 3/4 rectangle. These two will be placed on opposite sides of the tree, and termed north and south pieces.
Connect the north and south pieces by drilling screws into the ends of a 5' board at the corners, keeping a 90 degree angle. This is the west side. Connect the east side of the trellis with the last 5' board, making a whole rectangle around the cactus.
Measure the spaces beneath the branches, to fit in the remaining 10' boards for support. Saw divots into the boards if they rub too closely to the cactus. Space boards out at least 6" apart to provide sturdy support for the dragonfruit tree.
Screw in the lattice to the backside of the trellis to create a decorative appearance. You can lattice the sides and front as well if you prefer.