Is it worse than having to create endless functions for each type pair?
(b IntBox) MapToStringBox(f func(int) string) StringBox
(b IntBox) MapToBoolBox(f func(int) bool) BoolBox
(b StringBox) MapToIntBox(f func(string) int) IntBox
The T, U, and f names are the cognitive load here, because they are meaningless variables. For a specific solution, those would have meaningful names that would make it easier to understand.