Thanks Guys, i thought about it for a while to get the best result, knowing it had to support a fair bit of weight.
Terry, I laminated 2 pieces of Plasticard to get the thickness first. then i carefully cut the shape slightly oversized, making sure the inside arch matched the tyre profile, then started carefully sanding the outside shape to get the curve right followed by a taper and then rounded the outer edge.
So filing, check, sanding, check, filing, check, sanding.....constantly checking until I was satisfied.