Cut down a piece of rubylith and sandwich it between two bits of microscope slide, or even just stick it on one piece. The camera filters you mentioned will be even less safe than the 'proper' red filter.
It would actually be simpler just to make an under-lens swing filter using rubylith, a piece of plastic or plywood as a frame and a few cm of alloy strip as a support, attached somewhere convenient near the lens-stage.
I was going to suggest this. Or if you have a 6x6 glass slide mount, take the glasses from that.