A 40 can be used (still use 1) but be prepared to have issues with humidity and keeping the temps up. Not sure where you live but I'm from NY and we have like 30% humidity in the winter and it was a pain getting temps/humidity where it needed to be. If I were in your shoes Id can the idea of using any of those and go with a 66qt snap top rubbermaid for a baby or 96qt snap top for a sub-adult/adult... I have pics in my gallery of my set ups if you wanna check em out. When I first got into snakes I had a 20L and a 40 like you... Spent a butt load of money trying to get humidity/temps where they need to be and in the long run ended up switching to plastic tubs... the 66qt and 96qt are roughly the same size as a 20 and 40gal.