First homemade stir plate in a cigar box. Cost: $5 (the 1" stir bar). Computer PSU fan with hard drive magnet. Powered by another computer PSU converted to a benchtop PSU. Current output voltage to fan is 7V (10V is too fast; the stir bar spins off). Does this look like enough vortex for a 2L sample (in a 1gal. jug)?
And this vortex for a 1L sample in a 1L flask?
Yes you do not need to have big vortex. All you want is small dimple (1" or so) in the top of liquid.
Do you need to use a food grade stir bar? Or will any magnet do?
Stir bars are designed to be nearly friction free devices. They are Teflon coated for a reason! The bars with a ring in the middle or the dumb bell variety providing the least friction. Doubtful you’d ever get just any dual pole bar magnet to turn. Plus, a raw ceramic magnet could shed and sanitizing heat for any other type of magnet could damage the magnetic field.
Bottom line . . Just spring for the right device! 
I use a 1" stir bar I bought for about $4. It works great! I tried a paper clip when testing and it didn’t always work; I was worried. In any event, once I tried the stir bar, all was fine. I’ve never had a problem with mine, and I’ve made 10 starters with it so far.
What style stir bar are you using? The only thing holding me back from building a stir plate was that I’d have to get a big ol’ flask, but if there’s one that will stay centered on a convex bottom…
These dumbbells stay centered perfectly in a gallon jug. No need for a 4 or 5 liter flask. Problem is it’s $85 for 10 of them. A group of 5 brewers went in a couple of months ago to split up a 10 pack purchase of two each. Perhaps others here would want to do the same.
I have a 2" bar with the ring in the middle. It works fine in a cheapo 1-gallon Carlo Rossi wine jug with a convex bottom. This pic is just a 1.75 L starter, but it works fine all the way up to a full gallon.
Awesome. I put together the LM317 circuit tonight, and got a set of 1"-2.5" stir bars on eBay. Now I just need to decide which hard drive to pull apart, and find/buy a box to put the thing in.
One like this one.
After trying and failing to get good results with a HDD magnet, I broke down and bought a pair of neodymium magnets that are roughly the size of watch batteries and rated for 4 lb of lift apiece. Works like a charm now.
The photo is with a 1" stir bar with ring, running at about 6 V.
Ha! I love your container! Looks good.
I also have questionable results with HD magnets.
It sort of works but not that well 
I actually had a moment of embarrassment about using a clear container because of the blue LEDs in the case fan (while I don’t think I ever deliberately bought one, all three I had laying around had LEDs). But now I kind of think it looks cool, and spared me three extra solder points for a power indicator. :
Plus buying a new piece of Tupperware would have doubled the cost of the project.
I was surprised to find neodymium magnets at michaels craft store for only about a dollar each. I bought 2 two-packs and use two (stacked) on each side of the fan.
Edit: I guess it was a 4-pack for a a couple dollars. Here is the link.