There are a bunch of variations of this design on this site. But all our printers are calibrated slightly differently, and printing friction-fit parts can be frustrating - so this one adds an easy "slack" slider to help you dial in the best fit for your machine. You can also set the overall size however you want it, while still keeping the same amount of slack.
Thanks to tim1986 and Charles Rincheval for the original code.
Edit 16-Feb-2016: Tightened up the default slack from 0.5 mm to 0.4.
If it sticks, either file it out or increase the SLACK variable in the customizer and re-print. The former is fine if you're just printing one, but the latter is better if you're doing more.