Why not try a simple floating reed switch? I'm sure that the group will correct me if I'm wrong but I think that something like this would work
http://www.liquidlevel.com/collections/vertical-switches/products/polypropylene-single-level-float-switch-160?variant=302077465
Mike.
http://www.liquidlevel.com/collections/vertical-switches/products/polypropylene-single-level-float-switch-160?variant=302077465
Mike.