It curves out at the bottom to allow room for the bend in the plastic. The original fabric piece was actually longer than the piece I cut because the original piece allowed for a lot of bunching on the stand. I didn't want it to do that much so I cut it shorter. I also cute the piece on the fold which is important. I finished the ends of the piece and then I folded and sewed right side together using 1/4 inch seam and then flipped it right side out.
For the mobile pieces I wanted little birds so I free handed a pattern out of felt, cutting two pieces. I cut little wings that are petal shaped out of leftover fleece.