For sensors I'm using 4 stainless steel screws extending out the bottom of a plastic housing. They are spaced at the corners of a 5/8 square. I tested it with 2 screw and it worked fine but I went with 4 for the extra sense current.
I'm using 3 AAA batteries, my 2 AAA carbon battery statement was in error. When fresh the voltage exceeds the RFM69 input voltage specification.