Just because you don’t have a sump it doesn’t mean you can’t use an auto top off unit. Although I have never tried them myself due to the fact that I have always had a sump, two that come highly recommended are the AutoAqua Smart ATO Micro and the Tunze Osmolator Nano. Neither have dual sensors.
The AutoAqua Smart Micro has no moving parts which means that will not get jammed due to salt creep or inverts getting in the way. It also has an alarm that will sound and light up if the pump runs more than three times the original top off time.
The Tunze Osmolator Nano has a float switch with moving parts but this is covered with a protective guard. It features an auto shut off where the pump is turned off after 5 minutes of topping off if the original level has not yet been reached.
In my opinion, I would favour the Tunze Osmolator as it features the auto shut off whereas the AutoAqua Smart Micro requires you to be there in order to turn the pump off in case of failure.