You will need 3/4-by-48 inch plywood, having a length of 96 inches, to make the light box. Place the plywood on a table saw fence and make sure it is hanging 11 ¾ inches. You have to rip the plywood sheet lengthwise two times.
Now you have to cut one of the ripped pieces in two halves and you will get 11 ¾ x 23 ¾ pieces of plywood to make the sides of your light box.
From the other piece of plywood, you will need to cut two more 11 ¾ inches squares. Use a table saw or any other appropriate tool for cutting the sheets of wood.
Now you will need a mitre to shape the edges of the cut plywood pieces. First, tilt the table saw to 45 degrees and then place the mitre gauge at an angle of 90 degrees. This arrangement will allow you to mitre the corners of all the plywood pieces to 45 degrees but you must assure not to reduce the length of them in the process.
Apply glue to all the mitred ends of wooden pieces and take them to a flat table for assembling. Join all the mitred corners together and space two nylon strap clamps around them tightly. The excessive glue will be squeezed out. Let the glue dry for at least one hour.
In the meanwhile, you can mitre the remaining piece of 11 ¾ x 23 ¾ inch plywood using the table saw in the similar manner as you did before.
Place this piece of wood on the box after the glue has completely dried and the nylon clamps are removed. Make sure to glue its ends before placing it over the box.
Drive 1 ¼ inch pin nails through the top piece and affix it firmly to the box. This side will become the back of your light box.
When the glue dries completely, drill four holes at equal distance along the back of your light box. Make sure to drill the holes at least 1 inch from both ends.
Place the light fixtures within the box and screw them to the back of it. Pull the electric wires through the drilled holes and your light box is ready to use.