Raul,
Here is a simple mount setup. Get yourself some 1" x 4" strips of ABS plastic and a decent magnet cleaner or two. Drill a hole in the end of the strip for the float switch shaft to thread into. Then bend ABS strip 90*. Epoxy the strip to the wet side magnet cleaner. As mentioned in previous threads, I use a pair of redundant float switches for my ATO activation (If closed then ON//If Open Then OFF).
I also use a float switch for a "too high" level sensor and another for a "too low" level sensor. The "too high" shuts my skimmer off as well as shutting off my top off and automatic water changer and also sends me email and text alarm notification. The "too low" sensor shuts off my automatic water changer and sends me email and text alarm notification.
Those two switches don't need to be much different in height from one another and they just need to be positioned such that they are just slightly above and below the ATO float switches. Since your water level should never vary much more than a couple mm's, there is no reason to have any of these switches submerged to the point that the threaded shaft of the switch fully submerged where the wires enter. I've been running this setup for nearly 5 years now without a single failure.
This is my setup. Granted I am using the mounts that came with with switches in the link I posted above but you could easily make a mount using ABS and do the same thing. The nice thing about this setup is that I can easily adjust my water level by moving the switch up or down. They are easily removed and the end of the shaft is never submerged. This solution works perfectly.
Redundant ATO float Switch. Both mounted at the same level.
Staggered "Too High" & "Too Low" float switches. Mounts epoxied to a magnet cleaner.
The "Too High" and "Too Low" level switches in the sump. Never fully submerged.