I made one once that was tapered a tthe front: a little narrower at the end that fits around the back of the camera, and longer at the opposite corners. Like a dress. I don't know if it's ideal or not, but it allows me to move around more behind it, and I think it helps in wind. I did not use weights.
All the ones I have purchased were always rectangles. You could go the calumet site for dimensions.