I make my own out of silicon tubing about 6 mm in diameter.
I fold the front end back once to round of the part that goes in first.
Secondly I place a silicon ring directly under the head and sew it to the silicon tube both on the left and right hand side, like I said just below the head.
The ring measures some 20x 1.78 mm and by connecting it both on the left and right hand side the O-ring becomes oval and start to look like two little wings.
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