Whatever you carry
make sure it is not in conflict with your CCW. Here in KA, we have the CCW gun SPECIFIED by model and serial number. You DON'T want to have a different gun stashed that is not on the CCW.
For auto carry, it is hard to beat a combo locking hard briefcase with enough padding that the gun does not flop around. The case can be just snapped shut most of the time and in the event of being stopped, the combo can be spun to lock it absolutely. That way, you stand a good chance of their needing a warrant to inspect it and unless you are two wheeling away from a bank robbery site, they probably won't. Conservative clothing also is a help. At the very least, you will get a chance to have your lawyer's advice before the thing has to be opened.
Chancy carrying in car unless legal to do so. My favorite was a Stevens O/U .22-.410. Useful, but not expensive if a thief got it. And a very innocuous gun to have. Very un-threatening although a .410 will do just about any DEFENSE thing you can think of. Trunk was location. Now just carry the CCW and keep doors locked. I can get it fast enough.
None of this applies if you are entering a specific security zone. Especially if it is not public property.
Gun violations can lose ALL your gun rights. Gotta weigh the risk.