I buillt an ATO for my previous tank, I used 4 float switches and was in the process of swapping out one with an Optical switch for better reliability. If I had time I was going to build them but ran into cost issues, but I could redesign it to be simpler, battery back-up & less to fail and just help people make their own. I had mine set to a sump High and sump low which would shut down the main circulation pump if it went too low to protect it from a burn up and If it went high high (the second High switch) it would also shut the main pump off, then I had another switch in the coast to coast up top, if the water went too high there It did the same thing to prevent overflows. It had an alarm sound on it and topped off as needed, easy to make but a little elaborate for most systems, so that would be where a simpler design would come in.
On the last system, DonW made me a timer, I would program the run time, it would top off for so many seconds then shut off, probably one of the most reliable methods of topping off, you would have to determine just how much you needed to top off daily though.