How are you? The guitar case sounds like the perfect item to give a nice ozone therapy! I know how bad this kind of mutsy smell can be. I had more than one old pram come in that I had to remove from the house immediately because I started coughing and feeling bad.
Last week the weather/ humidity changed and one of my prams in the downstairs hall started getting furry under its dust sheet. DH told me to go check as the whole room smelled of camembert.
A couple of hours in the "ozone chamber" (a very small room with my household ozone machine in it) and my pram came back odourless. The white fur that usually sticks to the surfaces with a vengeance could easily be wiped off.
If you regularly buy vintage clothes, bags and (prams) items buying a little machine is a good option. They do wonders to the interior of the car, too. All bacteria and spores will die and you suddenly see a spooky pattern of dirt you never noticed before, but now can easily wipe away. The only problem is that the car as any other items has to be aired afterwards. My machine was around 80€ on amazone and is worth every penny! In case I´ll ever get a walk in wardrobe the machine will move into there!
PS: on your guitar case a vacuum cleaner with an attachment like this would do a good "brushing out" job post ozone. (Tried it on a Shearling coat the other week with amazing results.)
View attachment 4890963