High temperature RTV sealant. I've used it and it worked probably as good as anything else. I own a Char-Griller myself and can honestly tell you that the amount of gaps around your smoker, unless it's completely mis-aligned, isn't going to make that much difference, as several have noted at www.barbecuebible.com
in the chargriller topic (about 230+ pages of chargriller owners. The important thing is getting the smoke down and low to the cook grate by extending the smoke stack down via the use of some flex tube and using a Lowes shaker basket in the firebox to extend the time of your fuel. Several also have suggested using the minion method to start the char-griller and putting the lit coals on the left side of the shaker basket closes to the wall between the firebox and the main chamber and let it burn left to right to further increase your burn time. Hope these tips help.