I had tried with leather and other materials but the best of them was...EVA foam, yeah. I promise it actually works fine for it.
You can buy a lot of different colors of EVA foam, and this is very easy:
1: Cut it and get a Gear fit band shaped EVA foam.
2: Make a rectangle hole in the middle with 2-3 mm of EVA foam at the sides.
3: Put a buckle
4: apart of this, cut a rectangle with the same size with 2-3 mm of EVA foam all around.
5: Insert the Gear fit into the two holes, the band with the hole down and the rectangle up.
I think It is quite sturdy.