Put it where you want it, then be responsible and check it every day. No amount of serious oxidation is going to occur in a 24 hour period from sitting in a car if your firearm is properly oiled and ready for use.
I live in a salt marsh (think 100% humidity, 100+ degree summers, and more annual rainfall then Seattle), and have hunted in elements that other people won't walk out the front door in (duck hunting last year and a man was life flighted out of the same refuge I was in because he got struck by lightning). My working/hunting guns have been heavily rained on for hours at a time, but as long as I perform the proper PMCS on my firearms before and after use, I know they will stay it excellent shape for years of use.
Still worried? Oil it then vacuum seal that sucker in a bag that can easily be ripped open. That's how my toilet tank gun is set up. I check it and reseal every three months or so, never had an issue.
Y'all are ALL welcome for the toilet tank tip by the way.