As an alternative, you might consider constructing a filter from one or more layers of red cellophane or somesuch. I've done this to a couple flashlights that I use when I'm out with my telescopes. I used transparent red sheet protectors that I bought at Office Depot for a buck or so. Works fairly well with flashlights, might work well with the map lights.
It would probably look better if you could install the filter on the inside of the map light housing (don't know how difficult that would be since I've never disassembled mine) though the advantage of an external filter is that you can remove it if you need.