Swim usually just uses a clamp around the neck of the flask to hold it so the bottom of it just touches the top of the hot plate. This works fine with a stir bar. Swim has also used aluminum blocks with round impressions in it. In order to get proper temperature control, swim uses an oil bath with a temperature regulator (an electronic thermometer thing that hooks up to the hot plate and has a probe that goes into the oil bath and keeps the oil at a specific temp).