Welcome to the group! Your boa's behavior sounds perfectly normal; sometimes they roam, sometimes they find a spot they like and camp out, either way, if you have the appropriate temperatures available (it sounds like you do) they will find and utilize what they need. As far as housing is concerned, i second bcr229's opinion. Fish tanks with screen tops are popular but ill-advised, easy escape and terrible heat/humidity retention. When your girl gets a little older and bigger there are a great variety of reptile enclosures available but in the interim, to avoid spending a few hundred bucks every time she outgrows one, i recommend sterilite-type storage tubs, and get this, on their sides! Thats what i have for my 2 year old central american boa, its cheap, effective and offers much of the same benefits as the "real" enclosures. Sounds like your on the right path, enjoy your new friend!

*Note about pictures: since taking these, i have replaced the HORRIBLE analog thermometer and hydrometer you see in the cage with an AcuRite digital one - these are very popular b/c they are inexpensive but very effective*