Your observation is right on the $ :thumbsup: I am sure Leevalley from Canada has the hinges.Looks to me that the lid has two "fingers"attached to it that scoop the pen up. these fingers form part of the back of the box when the lid is closed. Look closely, you can see daylight under the pen.
I would build the box normal with a curved bottom from a router bit, then cut the fingers out at the scroll saw, attach to lid, attach lid to box. Genius idea.