Welcome. If I'm hearing your correctly, the sides will go in slots on the inside edge of the frame. Is that right? In that case, I'd start by making a full size paper template, lay out the frame made of wide material so the long grain is as close as possible to parallel to the curve. You will need to joint the frame pieces together, which can most easily be done with some sort of lap joint. That means laying out the pieces so they overlap enough so you allow room for a joint. Use the router to cut half way through each piece (so they overlap by the width of the frame). Then glue the joints together with clamps to hold it tight while it sets. Use exterior water proof glue.
I'd use some pretty wide material so you have to make fewer joints. You use the same pattern to make the second side an exact match. The top and bottom will be glued in place. I'd make a half inch wide rabbet aroung the inside of the frame, and bend the material around the frame, using some fasteners and glue. I would attach the wheels and other parts to the bottom of the finished shell with bolts (so you can remove it for repairs, etc. The door at the rear will require some fitting, and it will need its own frame, and a frame around the door's opening, to mount hinges and a lock.
The exterior will require some pretty rugged finish, including paint to match the motorcycle. And I think I'd add a panel across the front/bottom, to protect the ply from road dings and damage.
Nice project. Looking forward to seeing some pictures of your build and the final product.
