Someone once suggested using map numbers and level names from old classic shooters like Doom. It works because you'd have, at minimum*, 8 characters with numbers and capitals and occasionally special characters! So, E1M1Hangar, or MAP20Gotcha!, or E1L2RedLightDistrict, or whatever. And these old games with all their sequels and expansions had hundreds of maps, so there's never a shortage.

* E3M8Dis is only 7 characters, lol