Glass terrarium - 40gal. It's difficult to control humidity with these though.
PVC cage - I like the Animal Plastics T8. Depending on the size of the snake, you may want to use a divider at first. These enclosures are more expensive. I also prefer to use a radiant heat panel with these (also pricey)
Plastic Sterilite tub - 41qt.
These sizes may be a little bigger than the minimum you're asking for, but the snake will grow into them. They can be cluttered up if they're too big.
What ever heat source you choose - make sure you have a thermostat.