Friction is not the same between a canvas layer and an air layer. As an experiment, fill a bowl with sand. Level the sand with a scree bar. Tilt it until you

Friction is not the same between a canvas layer and an air layer. As an experiment, fill a bowl with sand. Level the sand with a scree bar. Tilt it until you notice sand shifting, and note the angle of tilt. Now level the bowl and put an inflatable bag on top. Tilt until you notice sand shifting and note the angle. Further experiment: put a vacuum hose through the center of the bag, and suck air out of the bowl as the bag covers the top. Tilt again. If your bag can make a decent seal against the rim of the bowl, and you keep that vacuum pump running, you can almost flip the thing without shifting any sand.