x2 with both of these comments!
There are several smaller localities of boas that would fit the bill. Hogs are great and male Dumerils are too. Even a male Colombian shouldn't get more than 6-ish feet or so.
You'll be fine with a 41qt tub for awhile if you start out with a baby, but any boa is going to need a larger enclosure as an adult. I personally recommend a 4x2x2 pvc cage from Animal Plastics (or similar company) for any adult boa regardless of locality or size. Boas tend to need a bit more room than ball pythons - they like to stretch out whereas balls tend to prefer cramped spaces.![]()