In the plant, we used a tool made from a phillips screwdriver with the tip rounded off and a washer brazed on it that allowed the retainer to stretch a bit before the washer hit the flange so it wouldn't punch through the tip; the retainers were kept in a tray on the bench under a heat lamp to soften them up, and were dipped in a 50-50 Joy soap/water solution just prior to installation. The guy on the inside pushed it through, and the guy on the outside grabbed the tip of the retainer with a pair of duckbill pliers and pulled it seated. Production parts were rubber, not plastic. Photo below of a homemade version of the tool showing the concept.
