I prefer not to advertise what I'm hauling out to the car, so I started looking for bags.
If you go to eBay and search for "sports bags" you'll find all sorts of stuff for storing your basketballs, jai alai sticks, and the like. Many of them will comfortably swallow several normal-length rifles.
Unfortunately, I wanted something that would hold my Mosin 91/30. It was a couple of inches longer than any bag I found, until I happened on a "golf bag bag." It's a bigger bag you put your golf bag in, basically. It holds the Mosin with a couple of inches to spare.
I have a sheet of 5mm plywood to make a rigid inner box, but for now I'm just using rigid foam and towels to keep rifles from banging into each other.