Nice work bobble! And greenmatter too, of course.
So both ends of the PVC bored out to 7/8" seems to do the trick, for both ends? The sensor and the cap, that is.. ?
Both ends bored out equally to 7/8". There is a lip on the green plastic part just below the threads. I pushed this piece into the PVC until the lip was level with the top of the PVC section. That created a very tight seal. You have to be careful not to push the top in too far though, otherwise the cap won't screw on all the way. Only push it in until the lip is level with the PVC.