Building the base of the Coroplast tailbox
First you need to create the interior of the box. This is a single piece that forms a saddle over the top of the rack, the interior sides, and the bottom of each compartment.
Technique 1: you can make very straight bends by laying the straight edge on top of the Coroplast and pulling the excess upwards. This puts a bend or crease all the way across your piece. This works best with the grain or across the grain- diagonal bends are pretty difficult. Fortunately, all the ”creased” bends for this tailbox are with the grain.
Cut a 24" x 36" rectangle so that the grain or flutes are parallel to the short edge. Crease and bend the two sides so they go down the sides of the rack. Crease and bend upwards 5 1/2" from each side. You should end up with a piece that looks like an "omega" sign:
Make two cuts through the middle two creases 9" long from the front edge of the piece. Crease and bend the center tab so it sticks up at right angles to the rack. This should allow you to slide then piece forward until it hits the back of the seat. Make rough cuts (scissors work) where the Coroplast hits the seat until you can slide the piece all the way forward on the rack.
Back the piece up about 1 inch (so there's space between the "tab" and the front of the rack) and temporarily hold it in place with duct tape. Using the outside surface of the wheel as a guide, mark an arc on the inside surface of each side, roughly 1/2" to 1" outside the diameter of the wheel. Remove this piece and cut away the excess, leaving a smooth arc along the front edge of the piece.
On the "bottoms" mark arcs from the outside edges 15" from the rear corner to the corner where the bottom meets the interior side. It's easier to see if you look at the plan. The radius of this arc is optional, but since the sides need to bend around it, gentle curves are recommended.
The pattern (click to enlarge):
The piece, cut, creased and placed on top of the rack (click to enlarge):
Note that the bottom tapers are not yet cut.