What about sandwiching a piece cut from a #29 deep red Wratten gelatin filter between two microscope slides that have been cemented/taped together? Someone must make a tape that can withstand the heat and if the filter is between glass how would it burn? Might melt but with no O2, would it be possible for it to burn?
Didn't Cibachrome filter sets include a red? What about a filter from one of those color correction viewing kits? Or, wait for it... a piece from a red welding curtain?