Poly- good points. This is an intra head filter (Leitz V35). I measured the temp of the light path where the red filter would go and it was 200F (per an old meat thermometer). I don't think (could be wrong) Rubylith is safe in a 200F+ enlarger head. I searched Roscolux and the red Rosco can go to 220F...that seems too close for me (ie fire hazard). I presume Rubylith isn't heat resistant (googling did find my answer in 3 mins so I quit lookin...).
Note that Pebeo glass paint is baked onto the glass at 325F for 40mins. That's why I knew I could get it near my enlarger head.
In the meantime of all this, I bought an extension tube for my enlarger that allows a longer lens to be mounted, now I have a much more comfortable working space and don't mind the under lens holder. Think I'm ready to call uncle on this project and appreciate the help. Now if Rubylith can go to 250F+....well you will for sure be the MacGyver challenge winner......