So if i am letting the yeasties in my starter propogate for a couple days, is keeping it on the stir plate covered with a sanitized bit of aluminum foil as a lid going to keep out contaminants?
Yes, that'll do fine, as long as you don't have a lot of fruit flies flying about.
No fruitflies. It's just a bit difficult for me to get used to after taking so many precautions to keep contaminants out of my wort when brewing. I did buy a little two-way air filter that i suppose I could plug in the hole of a number 10 stopper.
As far as stuff just floating around in the air goes, foil is fine. Nasty things just floating around won't get past it.