Sweet setup!
I wound up trying a "Danco #90" o-ring for the end of the plunger. 1/2" OD x 1/4" ID x 1/8" H That appears to be working out. I'm going to pick up some metric ones that are just a touch larger, M4x6, and see if those work too.
Really, I don't have any issues at the moment, but I...