Building the lid of the Coroplast tailbox
Now you make the lid of the tailbox. This step also binds the interior piece and the front piece together on top of the rack.
Cut a piece of Coroplast 17" x 22" with the flutes or grain running parallel to the short side. Put a crease across this 7" from one end. You should also taper the lid towards the back. The curving taper should match the curve of the bottom panel. The tab at the back will overlap the rear end by about 1 ˝”. This is where you’ll put your latch or tiedown.
Place this bent over portion in front of the "tab" and "horns" that are standing up vertically. Tape around the edges to keep all the pieces in alignment. Poke pairs of holes through all of the pieces and stitch them together so the zip ties hold all three pieces together. The joint should look like this:
The pattern (click to enlarge):
Here's what Piece #3 looks like. Note that the lid and the "tab" and "horns" are now firmly attached to one another.