These, but available in different sizes. And maybe make the hole 1/4" instead of 1/8" to allow for different inserts.
This would be a nice idea. It would eliminate the need for tank tread flap intakes if these were sold in various hardness levels (on a durometer for example)
Kind of like flex wheels but with flaps.
Has anyone ever actually used those
you can make them by cutting the flex wheels. expensive, but it works.