>>6874954Have you tried lego pneumatic tubing? Comes in various colors, lengths, and is easily cut. The exterior is roughly 5mm thick, but its hollow inside so you can put something inside it to expend the ends for a better fit. (like maybe snippings off a model tree) I love this stuff.
https://www.bricklink.com/v2/catalog/catalogitem.page?P=5102c26#T=S&C=11&O={%22color%22:%2211%22,%22iconly%22:0}