Newbie with a problem

Hello, I’ve lurked here off and on for some time, and it finally occurred to me to seek help with a long-standing problem here.

I’d like to replace the wood on a cast iron park bench. The metal frame is curved, and the slats ran lengthwise, of course, and fit the curved metal frame. So the slats have a concave surface and a convex surface. Some slats have quite the curve, maybe 5/8 inch depth over 3.5 inch width; some are almost flat.

The convex surface is not a problem, it can be shaped with a plane or whatever. But I’m stuck on how to shape the concave surfaces. There are 10 slats between ~3 and ~4 inches wide.

Any wisdom out there?

Thanks!