...
Place condensor lens on oil. Make sure there are no bubbles and carefully remove overflow oil from the housing.
Place a thin ring of quick seal Kwik-Sil (WPI) on the PMT housing around the lens
Add PMTMountPlate before Kwik-Sil sets, such that sealant squishes between the lens and PMTMountPlate. Be sure that lens does not move up while mount plate is being pushed down.