Anyway, I like my pillows to stay as unstained as possible, so I didn't want to use the one pillowcase that came on the wedge except as a protecting layer. I wrapped the pillow in an old sheet and used safety pins to secure it, but that was bulky and the sheet didn't coordinate with our current bedding.
I had this slightly shiny blue fabric in stash and was keeping it because the shiny part reminds me of water but wouldn't use it because the pale blue looks like healthcare blue and makes my skin crawl. The color does, however, coordinate with our bedding, so I used it to make a pillow case.
I started by measuring and wrapping. On one side, I traced against the pillow and added seam allowances, then cut and stitched. The wedge seam came out perfectly. I repeated on the other side. Somehow, the seam was no longer where I had traced it, at the bottom of the pillow, instead rising from the bottom to the top. And there was excess fabric across the bottom. At that point, I was out of inspiration and definitely didn't want to start over with carefully measuring and replicating the original pillowcase, so I turned over the raw edges, estimated an even hem, and used safety pins to take up the excess. I also used pins to hold the case closed because I didn't want to insert a zipper or have flapping fabric.
Not perfect, but less bulky than a full twin sheet pinned around the pillow and from the top, it looks alright. It will be easy enough to remove and wash, too.