"MacGyver'ed" Ventilators?

During a moderate to severe flu pandemic or other respiratory virus, there will be a critical shortage of ventilators. Due to labor shortages, it will be impossible to hand bag-ventilate all patients who require ventilator support. Ventilators are very complex and expensive machines. Therefore, It is not practical to stockpile such equipment. However, if a simple unit could be developed or instructions drawn up for a simple and cheap (plastic and tubes) vent device, it may be practical to produce and stockpile them or construct them using stockpiled components. Also, there may be alternative ways to automatically assist with ventilation on a large scale.