You really should reconsider leaving your instrument in a car- really.
Don't do it.
The best scenario I can think of is: Someone steals the bass (or the car and the bass) minutes after you leave them unattended and take them to where they will be safe from you!
Seriously- I don't wish that on you- but PLEASE don't leave your bass unattended in a vehicle for any length of time. It's what they call "the kids and/or dogs rule"- don't leave your bass anywhere you wouldn't leave a kid or a dog unattended.
Perhaps if you explain your circumstances you can get some advice on viable options that you might not have thought of...
-Leave it with a friend near by.
-Leave it at home and pick it up later.
-Arrange to borrow a bass.
-Have someone bring the bass to you.
Seriously, you say that you have "no choice"- you may just as well have NO BASS if you endanger it.
There has to be another way. Let us help you figure out what it might be- even if by PM if you don't feel comfortable spilling your guts on the forum.
PLEASE! For the sake of your bass...
Cause DIG IT: You can't play the bass if you aint got NO BASS!