It's too large for compases and I needed something larger than a dustbin lid (remember them, before wheelie bins) and nothing suitable was to hand.
Mt dad suggested I try the roof from a G gauge tram loco kit. He was right, it's pretty much spot on. Close enough for my freelance model anyway.
This made me think - if I took a piece of brass trip and feed it through my rolling bars, templates for any curviture would be easy to make.
I'll remember that next time.