I started to store them in my password manager for that very reason that months later I'd never remember exactly what I put down.

Then the next point is realizing since they are "bonus passwords" anyway you might as well randomize them and use password fields. (In the Advanced Fields properties of an entry in KeePass, for instance, you have a key-value store and when you add one the "Protect in Memory" treats the value the same as KeePass treats other passwords.)

The final realization was that these questions should probably be labeled "Phone Passwords" and in which case use something like Diceware that produces easily pronounceable over the phone passwords.