For years and years, my double bass (a german factory bass) was transported to gigs on the roof rack of my old mans car, in a gig bag and wrapped in a tarpaulin, tied to the rack with ropes and bungees. It survived the exposure intact.
Someone else we knew, who also transported his bass this way, made the mistake of driving his car straight into the garage with predictable results, but that wasn't a problem for us as our garage was always full of junk and there was no room to fit a car.
In those days the roof box didn't exist. Nowadays you might find one big enough to put a DB in.