Foil over the top or foam stopper. The yeast will do better getting oxygen during the start of the starter fermentation process. You also only need a constant stirring, not a huge vortex in the wort.
You can pitch the whole starter at high krausen, some prefer this, or chill for a day and pour off the liquid on top and pitch only the slurry from the starter.
I've done both and all were good.
I have done starters for my last 7 brews and I believe I have gotten better beer.