So my finfun tail wants to live anywhere to just below my bum to my knees. And I'm getting tired of pulling it up every time i surface. I bought the tail based on their size chart but its a little big when I 'm swimming (and its too late to return it).

What is a good way I can take it in or pin it to stay?

I didn't want to use safety pins because it may put a hole in the tail and its not quite big enough to tie a knot in it.