Just an idea, but, what about using Losi Mini-T or Mini-B hexes: https://www.amainhobbies.com/losi-minit-2.0-wheel-hex-2-los212011/p1144647?gclid=CjwKCAjw-b-kBhB-EiwA4fvKrKlml0ClDeFQQXOjyVPsnDapraSN0OdjBzOW_-98t8WbmkknY6347hoCe1oQAvD_BwE which are 8MM, and then using these 8MM to 12MM...