I would recommend a rubbermaid tub instead of a 10 gallon aquarium for a hatchling - for security against escapes and for maintaining a more constant humidity in the environment. I do not have mountain kingsnakes, so am not speaking from experience with them, and someone may know that the retained humidity in a rubbermaid tub would be too high for them, but I have used the ones that are 12" by 6" by 4" high for baby corns and kings and really like that the snake cannot escape. I have heard of too many close calls and escapes of hatchlings from other set-ups. We use these with the lid on and a security strap, put ventillation holes around the perimeter, use paper towel bedding and provide a tubular hide (cut lenghts of plastic pipe) that extends along the length of the tub. A heat source at one end that will give you the proper gradient for your species and ours have done very well in this. They seem to like the security of a darker container, the sides are sort of see though, but not as exposed as an aquarium. Ours stay in these until they are eating hopper size mice, then move up to the next size or into an aquarium.
Good luck with your new snake - they are very pretty - hope you will post some photos when you have him,