language agnostic - Involutary cipher (like ROT13) for an alphabet with an odd number of symbols -

rot13 has nice property of being involution a-z alphabet (26 letters), say:

rot13(rot13(string-a-to-z)) = string-a-to-z ; 

what simple cipher function alphabet odd number of symbols has same property? obviously, straight-up substitution won't work, i'm looking simple.

either exclude 1 of symbols in alphabet cipher, or supplement symbol not in alphabet.