A couple of thoughts: The flask is a sealed system. No mater how much of a swirl you have in the flask, there is no real mechanism there to draw anything into the flask. In fact, the yeast produce CO2 which will actually cause a positive pressure inside. This has been argued in detail before. The biggest thing that a stir plate accomplishes is keeping yeast cells in suspension & in close proximity to the sugars (Food). The swirl also helps the CO2 (Waste product) escape more efficiently. You should aerate/oxygenate your starters. As far as contamination goes, be assured that foil is fine. Microbiologists use foil all the time for their cultures. The stuff you are worried about doesn't have legs or wings. It is at the mercy of gravity & air currents, so a foil cap is a barrier that they don't have the means to get around.
You should go fishing; it is good for the soul. Have you ever heard of a single Nazi who liked to fish? You can't trust people who don't fish. We need to teach our House & Senate members to fish. Those who don't want to should be used for bait.