I did two last summer, existing units that the lines had stopped up. But they had an 1 1/2" on the pan to put it on, used a bulldog float switch.
Don't see an easy way to fix one with 1/2" gap by pan, and don't think wet switch would work in base pan of unit.
Hard way would be to put second sheet metal pan under main pan and put switch on that.
questions asked, answers received, ignorance abated