UPDATE:
I was successful in glassing the box by wrapping the 50″ wide fabric around the sides although my length of fabric was unable to make it all the way around. In anticipation of running short, I glassed the majority of the back panel (to within an inch of the edges) while glassing the bottom of the box. This was not the most efficient use of fabric as the box is only about 18″ tall (and using 50″ fabric), but it was necessary due to the tapered nature of the box. The result was a clean, uninterrupted wrap all the way around with the only overlap being on the back, which will never be seen. I made 4 small darts in the fabric to make the corners on the top edge and pushpins were not required.