A base 30 time stamp will give you about the right number of characters, case-insensitive, and without the possibly-ambiguous 0, 1, O, I, L (lower-case ambiguous) and Q (possibly upper-case ambiguous) using 23456789ABCDEFGHJKMNPRSTUVWXYZ as valid characters. Adding back the 0 and 1 (that is, anybody listening to a user on a support call, or any code, would automatically interpret I or L as 1 and O or Q as 0) gives you base 32 and a shorter ID code. If milliseconds are going to be too collision-prone (that is, you can't recuperate the time you'll occasionally have to advance), a short entity code (user or server name abbreviation, f'rinstance) prepended with a dash should make it memorable and user-friendly.