Theyre a bit rough but they work:

The elastic bands from my current 5mm ones fit perfectly too Next plan: make a range of sizes, order them then list on Etsy (for more than the $1.50 they cost to make :p )

Shapeways also do a “co-creator” system where someone with knowledge of the 3d software (i.e. Me) designs something that can be customised on request from a peon (i.e. the rest of the internet). I might do something similar with these, I’m sure theres people that want custom tunnels but cant figure out how Blender/Autocad work..