I work with Gray Banded Kings, Durango Mountain kings, Variable Kings and Mex Mex with adult size between 3' and 4'. I feed the adults in 12 quart Rubbermaid latch top tubs which measure roughly 9"wide by 12"long with nothing on the bottom, they feed very well in these small tubs and are as happy as a lark. As long as there is enough room in the container so that they can stretch out a portion of their body enough to easily work the food item down, they have enough room. Your Cal King will reach a much larger size eventually so you may want to consider the next larger tub. These things are great... they are cheap, the lid latches securely and it ventilates well enough around the rim for feeding without having to make any additional air holes. Plus... when you are not feeding you can store some of your snake keeper stuff in there!
Mike