Apologies for the pics being on their side. I'm getting better - my previous pics were upside down !
What I've found is the wheel mounted in this way is extremely secure. Previously, when just sat on the substrate then I thought there was a lot of wobble - but i do have several inches of carefresh beneath.
He's actually taken to burrowing and sleeping in an area beneath the wheel - he'd be unable to do this I reckon if the wheel was stood on its base.