This is slightly OT, but I just made two super cute ones -- an no, I am not a great seamstress.

My local fabric store (Joann's) sells fabric that has already been gathered with elastic at the top 10-12 inches. I just measured around the largest part of my chest to get the number of inches I needed, and the gathering made sure there was plenty of fabric to cover my third-trimester belly.
A quick seam down the back, a hem (which you could even iron in with that fusion tape or just fray up an inch or so for a different look), and wide ribbon straps to hide my bra straps = a super cheap, cool, cotton sun dress.
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