Thanks, ayjay! So I could do it with cardboard, right? And just give it the approximate bend I will have?
I'm not sure that that's how I'm seeing it.
I'd make the lid too large and then trim it down.
Roll out clay, put over bowl.
Make a flat cardboard template, cut a hole at the centre which is the finished size of the lid, put over bowl (and clay) and trim lid to size.