I had a featherweight case that had the dreaded musty smell. Tried many suggestions for getting rid of it. Soap and laundry softener sheets were two of the worst recommendations, as they simply added cheap perfume overlays to the musty smell. Leaving it open outside in the sun for a day did nothing. What finally worked for me was placing a couple of Dr. Scholl's charcoal OdorEaters inside the case and leaving it closed for a month. Worked perfectly. After that I just kept one of the OdorEaters inside the case with the machine to add cushioning and make sure the musty odor wouldn't come back. It's been many years, and the case still smells fresh.
What I liked about the OdorEaters is that the charcoal is contained so I didn't have to worry about cleaning up spilled charcoal. Although you could use loose charcoal, there's the possibility of having to clean up a mess afterward. For a large trunk what I would do is go to a hardware store and ask for charcoal filter material (used in air cleaners, air conditioners, etc. to filter air and also to filter water). I think it is sold in rolls. This would probably be cheaper than OdorEaters and just as effective. My best guess is to leave the trunk closed up with the charcoal inside for at least 2 weeks.