It's two ways to manufacture this lid.
Natural - one thick piece of stock
And economy - set of flat arcs glued to stock
If you choose 1st way, you need to machine inside lid by long tools - at least 42mm
The minimal diameter of this finishing tool i seen is 6mm, but you can machine with 10-18mm diameter, depends of your spindle power.
If your Rhinocam license allows 3 axis advanced strategies, you can use horizintal finishing directly, without prior roughting.
Also you can flip stock by defining new setup with one axis rotated at 180°. Very useful for me.
And you also don't need to make bridges. Just make stock little thicker, after you process both sides, you can manually sand out thin piece of stock below