I've built quite a few stir plates for myself and my brewing friends over the past couple of years. I don't use the HD magnets as I have found them to be difficult to position and balance. I use store bought rare earth magnets. IMO, a 2" (51mm) stir bar works the best of all that I have tried, and I've experimented with quite a few different types. I've found that the larger 2" bars do a better job of stirring at relatively low rpms. I also like the stir bars with the center pivot ring. The ring reduces friction between the bar and the flask making it start more easily and less prone to stalling. A stir plate with well balanced, aligned and spaced magnets should run quietly and seldom, if ever, throw the bar when operated at a reasonable rpm rate. I have not tried the oval (football) shaped stir bars, but one of my friends uses one and likes it a lot. I've also used the barbell shaped stir bars and they work well, but they are generally pretty big and require more power to turn. I like to use the barbell stir bar with a one gallon glass jug with a slightly domed bottom. I've also found that it is best to match the position of the magnets a specific stir bar size for optimum performance. You can check your stir plate for balance by running it full speed without the flask or stir bar. Any vibration you can feel should be minimal. Strong vibrations indicate poor balance of the magnet(s).