Perhaps the safest auto-topoff would be to determine your evaporation rate and use a dosing pump to deliver that amount back to your tank over a 24hr period.
After that, I think a float valve from a reservoir with a limited amount of water would be next in safety. You just want to make sure that when your auto-topoff goes astray, which they will from time to time, that the amount of water does not nuke your tank by dropping the SG too low.
Float switches are just not dependable, IMO. I have used them, but will not chance it on my new system...if I ever get it done.
dave