Some cages offer the addition of flip top lids that open the entire top of the cage in addition to doors, which provides another easy access point. (northstarrescue.org)
Two questions though — I only have one flip top bottle — could I use mason jars instead? (mommypotamus.com)
If you don't like plastic, glass jars are always a good alternative, either the mason type of flip top. (thehealthyfoodie.com)