Jasper, once you have the frame, the easiest way to pattern the first prototype is to draw the frame on the bed fabric, then decide how the sleeves should run. This is a good question, and one i will try to answer more throughly with a DIY step by step pics. Sleeves are between 6” and 7 1/2” wide before sewing, depending on tube diameter.
Seam tape all curves before sewing into sleeve.
More soon.