The lid can be as simple as using some clamps around the lip to hold down some aceyllic or wood. The melting point of the plastics is generally higher than your basking spot will ever get. My small cages are all PVC, which generally has a melting point of 140 degrees F and I have never had an issue. plastic stock tanks I believe are ABS which also has a high heat rating. All that being said. In my 8ft cage I have now with 2ft of substrate there is no need for a mist king. I use a watering can and after the initial period of dialing in my humidity, I have had to use a full can once and my humidity has stayed up with no issues.
Scroll down to view lid on a trough: http://www.repticzone.com/forums/Mon...es/945676.html