Fiberglass Utility box.
The lid can be tilted back for better aerodynamic drag ratio.
The lid closes to keep out the rain.
It looks like this from the front.
This is the view from behind.
A close-up of the mounting hardware, made from 1"x4" polyethylene stock.
The frame is a Worksman Y3K. Worksman is known for it's utility bikes, but the Y3K comes without a utility box. The Y3K is furthermore out-of-production.