Mesh wheels are probably the best wheel on the market. they are available in large sizes, are quite cheap and are easy to find. They are also available in a variety of colors, and can easily attach to the roof of the enclosure. The only downside is that if one of the wires broke of, a gerbil may step on the sharp wire and get hurt. That rarely happens, though wheels must still be inspect daily just in case. They also squeak unless lubricated with vegetable or olive oil.