I thought that a product like that might cause a problem with the foam backing, but a chair (and a car seat) uses a similar material.  There are only about 8 small screws holding the fabric cover on, so that is not a big deal either.

Hi Andrew.  Funny that you should mention rain.  Heard Angela and Jennifer Chun last week at a world premiere of a dance choreographed by Jonah Bokaer (Late Nights on Air) and I swear their instruments were in Cecilio vinyl cases.  At first I excitedly hoped that the violins were Cecilio as well, since their tone was excellent.  That hope was dashed when I learned via the internet that they play valuable antique instruments.  Perhaps that night?

All of their album covers seems to have photographs of them walking in the rain with their respective violin cases exposed to the full down pour (more credence that they might be Cecilio violins).  Hope springs eternal.

A vinyl covered case, as well as fiberglass and carbon fiber, should protect from rain quite well.  I have seen photos of expensive cases with a removable cloth envelope.  It would be fairly inexpensive to provide a cover of gortex cloth and taped seams to provide excellent water proofing.  And your idea of a plastic bin bag should also work.

Goretex sheds water but passes water vapor.  I use it for diy violin case humidifiers.  It may also be possible to use a waterproofing spray commonly used on outdoor jackets.  

Hi Andrew.  I have almost a year’s experience with the use of a goretex fabric “window” on my diy case humidifiers.  Goretex definitely passes water vapor, since the gel spheres all greatly shrank over time in the case.  Liquid water also beads up when spilled on the fabric’s surface.

I had hoped that the pores in goretex would not allow the passage of bacteria.  I don’t think that is the case.  I wanted to reuse the gel sphere by re-hydration.  Bacteria appear to be using the spheres as a food source (like a bio-reactor) and I may need to utilize some method of active biocide in the humidifier.