arbitrary base conversion from base 10 using only builtins in bash

I'm trying to sort out a bash command to convert a number from base 10 to an arbitrary base, using a specified set of characters (for example, to base 26 using letters a-z, although that's not actually my use-case). I *have* solved this problem before, but not in bash (which I have limited experience in), and not in quite some time. Any ideas?