To get engraved plastic sign like infinity posted, you may have several options, sign company, trophy/awards stores might be able to make or order them, some electrical supply houses may even be able to make them or any other signs/placards you may need. Those that do make such things sell a lot to industrial customers or clients that do work in such places, and an electric supply house likely more familiar with needs for an electrical contractor than a general sign company or a trophy store would be. Tell them you need a plastic sign for a HAND OFF AUTO 30mm selector switch, and I bet they have good chance of getting you what you wanted on the first try than those other places do.
Otherwise what is wrong with a printed label from Panduit label maker or P-touch label maker? Both will withstand weather fairly well, and user won't ever understand what it means anyway. If their cordset for their generator plugs into it they will use it regardless. If they change generators and move or make adapters for the cordset - still won't matter. This is somewhat pointless requirement that is intended for the ignorant that will not comply. Any "qualified" person that gives a **** will already know what to do when changing or setting up a new arrangement.:blink: