When I finally decided to just make a cover right now, I figured I didn't need a pattern, just a few measurements.
I had some quilted fabric that was left over from a sad, clearance bedspread. This was not a bargain - it was very cheaply made: it was basically backed with interface fabric, which melted in the dryer. And the pugs did not like how it smelled. So it got repurposed. I used the fabric to cut out two sides and one main piece.
I stitched around all three pieces before joining, so the top-batting-backing would not shift too much. Then I stitched the sides to the main piece. It fit perfectly. This was a fast, who-cares project; lining not necessary. But I did finish the bottom edge with some craft ribbon that has been hanging around for a long time.