Not that this matters anymore, but the directions for the mist king say to drill at 5/8" hole for a loose fit and 9/16" for a tight fit. I'm going based off memory here. Either way, I have always used the tight fit option and never had leaks. It's a PITA to get in the hole tho so just be patient with it. Hope that helps.
When my first Mist King timer broke after I dropped it in water, I spoke with Marty at Mist King who sent me a new timer for free. The second timer lost a little silicone button after having it for a few years. I took it with me to a reptile show and the guy that usually works at those (whose name I forget) replaced it, again, for free. In my experience, I can say that they are very good about making the customer satisfied. Good customer service is always better even if the product is a bit difficult sometimes