If your magnets are strong enough it will hold the straight 1" bar. I use a flask but I also have a growler, a 1 gal carboy, and a 1 gal. glass pickle jar all with concave surfaces and I can spin a straight bar in all 3 containers and pull a 2" vortex with the containers half full. I use (4) ceramic 3/8" magnets all touching each other in a straight line and they are right up to the bottom of the flask when I'm stirring. My stir plate has a 2.75" hole in the center which allows me to see the magnets and make total use of the pull force of the magnets by not having to pull through whatever your box/ plate is made of. I raised the magnets up almost touching the glass by stacking up layers of double sided tape and sticking the magnet to that. I originally was using only 2 magnets and couldn't get them to hold. I almost pulled the trigger by buying (2) 3/8" X 3/8" neodymium magnets but then decided to use 4 magnets instead and it works beautiful.
So if I was you and didn't want to buy a flask, I would mess around with the magnets because your idea of using a straight bar in a growler can work.
All I ask is three beers apiece for each of my co-workers...I think a man workin outdoors, feels more like a man if he can have a bottle of suds.