Auto-topoff for a sump-less tank?

[Richer]

I am currently cycling a 30 gallon sump-less tank and manual topoffs aren't a major problem. However, since it is a rather small tank, even a little bit of excess evaporation can affect salinity in a noticable way. To make things even worse, I will be leaving the city for an extended amount of time and I cannot get anyone to come in to manually top off my tank. Is there anything I can build/buy that will automatically top off my tank?

-Richer