Step 1: Materials
There are many sizes of bottles. You will want to pick a bottle with little grooves and little bumps, to minimize distortion. Perhaps a 1.5 liter or bigger, but not too big!
Step 3: Tape!
Tape around edges and bottom, so that the bottom curves towards your chest.
Step 4: Breathing Tube
First, you will need to get a decent sized tube, and drill a hole the same size as the tube in 2 caps.
Next, tape it until it is sealed around the sides.
Step 5: Filter
First, take a bottle and cut off the top, and put a filter on the top.
Next, tape it down.
Make more if you will be using it often.