Driving to or from your favorite brewery with a growler or two can be a bit of a pain. They roll around when they are empty and tip over when they are full. I stumbled across this method a few days ago as I was on my way to get some much needed holiday beer.
Step 1: The Setup
Set the growler down in the middle of the car seat with the handle facing forward. Buckle the seat belt normally. In a perfect world, this is all you would need to do but if you leave it like this the growler will escape and roll all over the place.
Step 2: The Loop
Grab the diagonal portion of the seat belt and pull out some slack. Make a loop with the slack and insert that loop through the handle of the growler.
Step 3: Threading the Loop
Unclick the seat belt and thread the buckle through the top of the loop. Make sure you keep everything in front of the growler while doing this.
Step 4: Securing Your Growler
Pull the seat belt buckle through the loop and click it back into place. Your growler is now secure.
I've also tried this with multiple growlers, you can get two on the same loop comfortably. They might clink into each other as you drive though.