As the title says, it looks like a pillow case and could potentially double as a pillow case, it's actually a hatch-insert that converts the "sitting area" and the "leg room area" into two separate compartments. The bottom compartment is intended for water bladders and heavier tools, the top is for foodstuffs and lighter supplies, and my camping equipment will go behind the seat.
|Not drawn to scale. Original graphic available here.|
It's a 2.5 layers of cardboard held together with duct tape, with the "grain" of the cardboard in different orientations to strengthen itself in every direction. The cardboard is wholly recycled (one side a frozen chicken box, the other a pair of snack-bar boxes, with frozen pizza circles as additional braces, if anybody cares).
The zippers are rescued from a pair of old cargo/chain pants that are falling apart without my help, and the orange fabric (a bit of a shame I didn't have any large-enough black pieces) is from my store of scraps.
The black bar is another piece of cardboard-and-tape (from a recycled chopsticks box) that will serve as a center brace for the insert, providing additional support. It's been worked into a triangular cylinder (and took about 5 minutes to hack together), and is attached to the front crossbar (via tape) and the underside of the seat (via a tension strap built into the trailer).
The longest part was assembling the cardboard and getting the pieces lined up the right way. The most difficult part was getting the zippers lined up and attached. I really hate working with zippers.
All told, it was about 8 hours of work.