Is there a symmetric cipher that is explicitly designed to be slow?
A memory hard PBKDF (or password hash, same thing, different name) without memory hardness is basically just a repetition of a hash or cipher.  In that case you might was well be the repeated application of a block cipher (with an alterated key if needed) or - indeed - increase the number of rounds.

The issue with increasing the number of rounds is that you also need to extend the key schedule. But since you're not trying to achieve more security, you could simply repeat previously calculated keys (if the key schedule itself cannot be easily extended).

Note that bcrypt is a password hash that uses a block cipher rather than a hash function. You could check if that suits your needs.