Hey Gunter,
Believe it or not...it is simply about 5/8 of an eight inch circle. You just have to elongate it a bit for the width, or the cover will be short. I think this had a 2 inch gusset as well. Very simple design. I suppose I could draw up a pattern if you want. Don't know how THAT would turn out. A lot of my stuff comes right out of my head...and I don't think to draw it out till I want another one. I am getting better at that though.
Have a good one,
Rick