This is all very iffy.
Cut a square of fabric about 12". (I made two.) I used some curtain swatches that someone had once given me for patchwork but they were too heavy.
Sew edge to the center point and stop about an inch from the opposite edge. Do the other side the same.
Clip corners (Well, not the center corner) and turn inside out.
Fold the unsewn point triangle in and insert an 8" piece of string or cording with a knot tied at the outer end.
Finished! Ten minutes top!
Fold the edges in, roll up the triangle from the long end and slip the cording under the last roll of cording. Pull tight if necessary. No real tying is needed.