Newbie question - how do you protect from your stock gapping down overnight?

I purchased 400 shares on ANGO - looking at all the information it seems like a good candidate to break at least up to $112-$113.

But now I'm in the unenviable position of holding 400 shares into extended hours. I'm on thinkorswim which means no extended hours stoploss - if this thing drops a dollar or 2 I'm going to wake up to a very unpleasant surprise.

The stocks we swing trade generally have greater than average volatility, so that seems like a fairly real risk.