Bill the guy at the photo shop checked and a 67mm filter will sit in the holder.
A new 67mm filter would be about $40, so he recommended either a used 67mm red or look for something more reasonable in price.
I googled "rubylith" since I heard about it. But it turns out that is a film, not a hard piece of plastic. So it would be tricky to put into the filter holder w/o something to hold it from sagging.
The Kodak color printing filter looks like it might work, but I don't need a piece that big.
I'm also being cheap on this repair since it is not a "have to fix" problem.
I can't believe no one pointed this out. The red filter for a camera is not necessarily the same as a safelight filter for an enlarger. The internet is littered with posts from people who tried to kluge a safelight from some red material. You need to get something that is specifically made for safelights. The Kodak filter in the earlier link is for COLOR printing. It does not say it is for a safelight for B&W printing. Also the picture says it is for illustrative purposes only. The shade of red may be quite different.
As others have said I don't know what the purpose is for those filters. If you look at Kodak's safelight instructions they clearly say direct lightl should be at least four feet away from the paper. A lot of us go through great pains to skip direct lighting all together and have a mostly indirect lighting setup. And when when do pull paper out for the final print we use our bodies to shield it from even the indirect light bouncing off the ceiling or whatever. Why anyone would want to stick a piece of paper directly under a safe light at a ridiculously close distance is beyond me. Maybe if you are printing without an easel and want to lay the paper in the right spot.