Snow might be a problem. The slot looks different to Joe's. The 'widening' you describe was actually my successful attempt to put an exact parallel circular slot track in a very roughly cut un-parallel foam slot. It does run inside. But it has an overhanging weather lip.
Wheels run on the top curve of the slot.
Colour is a contentious issue. The green was hot to touch on the first 25 C day I had, and at that stage I was worried about it all cracking due to the heat. Green it would stay. It's now green. The wall is 15 steel supports on a radius matching the dome base ring centreline, set in concrete.