You could run a bead of silicone all around the edge where the 2 pieces join and then put the rubber surround back on. I’ve done something similar with an open/close sensor, but it never gets submerged.
Those presence/arrival sensors do go through batteries faster than any other device I have, and having to clean and reapply silicone would eventually get annoying.
Without a case like in the post above, you may struggle with completely waterproofing the device. You could investigate looking into the floats used by boaters for their boat keys. I have a couple, and they’re designed to be relatively small. The one I use screws shut and holds my registration card. It’s just the right size for the arrival sensor.
Here are some ideas: