A smaller cage is fine if you don't mind having an extra cage around or can reuse it, everyone is simply attempting to save you from money and effort. There's honestly not much setup, in my pvc cages that size there is flexwatt heating underneath just like a heat mat and also a radiant heat panel. No matter what size cage you end up with the setup will be the same, place thermostat probes, set temps, adjust to what they need to be and check them regularly. It's just a whole lot more difficult when you are attempting to retrofit a glass aquarium. In a glass fish tank heat doesn't hold well, unless your ambient room temps are quite tropical you'll likely need to add a heat lamp. Those are riskier and harder to control temps on. They also suck humidity like crazy. To solve that most people cover all the air flow (already poor in an aquarium) and either mist very frequently or keep wet substrate. That's upping your chances of a respiratory issue. Stability and security is what your baby boa will need. Get what's best for the animal, not for yourself.