Anyway, I needed a cover for the form and wanted it to be as sleek as possible - no tassels or buttons. I couldn't justify the cost of buying a pattern, especially when I couldn't find one that was exactly what I wanted, so I measured and drew and made my own.
I didn't have a compass, so my circles weren't exactly round, but good enough for a first try on a project I needed to get done more than I needed to get done perfectly.
I used the same open closure as on a pillow sham, but then it gaped, so I ended up tacking it down in a few places. It's not convenient for washing, but it'll do.
I thought I had a photo of the three pieces, but can't find it. This is two circles and one rectangle. The only tricky part, other than measuring and drawing the circles, was easing the circles into the rectangle.
I marked the "center" on the "top" and "bottom" of the circle. On the rectangle, I pinned the open edges so that they were in the overlapped position they needed to be in when sewn. One "center" of the circle was aligned with this line. I had also marked the center of the rectangle edge and aligned the other "center" of the circle with this mark and was good to go.
In the created-in-30-seconds-using-Paint image below, the red (and the sneaky orange) marks go to the red marks and the blue marks to the blue marks.