Thanks Chris, sounds like you have the tools to make the pattern. Get the shape you want and then unwrap it. If you can print it off you can glue the paper pattern to the steel and cut around it. If it's close enough you can panel beat the last bit.

I cut it by hand and marked the points out by hand, wish I'd taken some photos of the making to show you. Before cutting up the steel I made some paper models, printer paper's a fair bit cheaper than big sheets of galv steel.