Step 2: Build the Display
To make the grid, cut 8 identical 2x10" pieces. Then cut 4 equally spaced slits into each one. The slits should be 1" deep, the same width as the foam board. Interlock the pieces to create the 5x5 grid.
Next, cut a 10x10" piece of foam for the back. Hot glue it to the 5x5 grid.
Cut 2 more 2x10" pieces to form the sides, and glue them on.
Finally, cut the 2 (10 3/8)x2" pieces for the top and bottom. My foam was 3/16" thick, so I had to add 3/8" to the length to make them long enough to cover the sides. Hot glue them on.
Note that in the photo below which includes the top & bottom & sides, the LED's are already installed and wired up. This is because I didn't attach the top/bottom/sides until I was in the testing phase, but it's easier to deal with if you glue them on earlier in the process.