As far as I know, there are no examples for #[rkyv(with = Map<...>)] and family, neither in examples directory nor in the book. And from existing examples it's very difficult to actually write one's own wrapper, I was unable to write very simple wrapper for chrono::NaiveDate. This example cleared a lot for me: https://gist.github.com/djkoloski/44ec7ba245c5b26d0949218845aed16b, perhaps it could be incorporated?