Thanks Marla! I found some brown fabric in my stash that worked great to slide the rubber through.

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When I flipped it right side out, I found that I had a little bit of bunching on my side fins but I thought it made them look ruffle-y.
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And Then I inserted the monofin. It makes the tail SO LONG.

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Also I would just like to remark that, even though this stretches, I was kind of really excited when I had to take it about 1.5 inches.