Safe wheel with mesh track and no crossbar for tails to get caught on.
The Stealth Wheel is an original homebrew design exercise wheel that works well with small furries such as sugar gliders, squirrels and others. It can be purchased pre-built to make it easy for the new owner but you can also make one yourself using simple to follow plans. It simply uses two bucket lids sandwiching a mesh track and is suspended on a bearing. The neat feature about this design is that it is open which makes viewing the animals easier and it is also hubless which allows the animals to jump.
You can find the original Stealth Wheel listed HERE or via the web site.
Also check out the similar Raptor Wheel.