Cut one piece of 2-by-6-inch lumber to the desired length. The piece of lumber should be longer than the photo mat being cut.
Attach the large aluminium L-bracket to the wood using a clamp. The longest part of the bracket should be in the center of the wood. Drill a 1-inch hole about 1-inch from the bottom of the board. Drill the hole through both the bracket and the wood.
Cut the small L-bracket into two pieces using a saw. Each piece should be the same width and length as the wood.
Place the two L-bracket pieces on the board with a clamp. Use a T-square to make sure the brackets are at a 90-degree angle to the large L-bracket. Drill a hole through the brackets that were just attached. The hole should be large enough for the smallest bolts to fit through.
Draw a line along the center of the board in the middle of the smaller brackets. Cut a trough along the drawn line using a router.
Insert the small bolts through the wooden board and small L-brackets. Tighten the bolts once inserted. Insert the larger bolts into the large L-bracket and tighten.