the rod type sensors are very good, needs a hole about 1 1/2 inches in diameter for the float to pass through, you get a gasket and some self tapping screws and the wiring is easy.
Just buy the length of rod to suit the depth of tank.
I've not fitted one as I cannot get to the top of my tank, and cannot move it either - at the moment.
A thought occured to me. As it uses an air pressure to operate the sensor, why can't it be fitted externally to a T connector. The same pressure as is in the tank is apparent outside the tank when connected to the outlet.
I will contact the company to find out if this is feasable. If so, I may fit one myself.
That's my basic option, but as I have a motor sailer, I would like to have the readout/indicator up on the drive position. I can think of no reason why the electronic/pressure unit cannot be fitted externally.
I will contact the company mentioned and put it to them.