When you say you tested with a meat thermometer, my guess is that the thermometer was mostly radiatively heated by being in the light path. The filter may be less heated maybe. The rubylith I have is plastic of some sort, but 200F isn't *that* hot... Don't blame me if you burn it down though

Anyway, rubylith is real thin. You could stick it right under the lens or slap a large sheet of it directly on your print?