I have used clear plastic medical IV-type tubing and really liked it. What I have been using lately is very fine surgical tubing that is sold in model stores and used as gas lines for radio controlled planes - it is excellent and fits perfectly over the hollow Q-Tip tubes and with a little work over the brass tubes. You may want to check out a plastic shop to find different sizes of plastic tubes - though I have found the sizes I need at the same model shop as above where it is used as a cable housing for gearshifts (I haven't found a use for the braided cable that comes with it yet!).
Tight lines - tyler.