I second making your own. At that size it will be relatively easy and a good learning experience. Most local hardware stores, even smaller ones, will stock and cut glass thick enough to hold one gallon of water. (I have worked at a few hardware stores and every single one has been able to cut glass.) Plywood is an option for 4 out of the 5 sides if you want to make it even easier. And if you plan on making several, it will end up cheaper in the long run.