I mentioned this to my wife, and in about 15 minutes we had come up with about 10 other things to use other than sharpies. She generally buys the school stuff for the kids, and apparently we have a very poor selection of Sharpie colours in Australia, so I personally would look at using something else, then printing an attachable system to the back of the machine, ideally small enough to clamp on the tubing or similar. Going to run a bunch of tests this week, will post pics with diff materials and diff plastics. I would assume clear and white work the best.