I tried to find pictures of boxes with rounded corners and they're all rounded on the vertical corners, like a box of mints. Or the polar opposite of a bread box with a lid with a >4" radius of curvature on the lid.

But look at the lid on a metal tin. The lip around the edge is often not a perfectly sharp bend. There's a small radius to it. So imagine the 'lid' was a single piece and the other sides were flat pieces that bolt together. If you size the pieces right they should fit into the 'lid'.