I used fairing bearings. I made a video about them, if you want to watch it. It uses a fairing with an open top as the casing the wheels roll inside of. What's nice is that because it's made of fairings, the shape is customizable; the fairing i used was very flat so it could fit inside the tank.
If you've got any ideas, please let me know. I've sorta lost interest. So far I've got suspension, a working turret, and working gun (not shown in video). https://www.youtube.com/watch?v=_Yi6NcpUKLw