Oh, I should have commented on this thread ages ago, when I first found out about breathplay I used a silicone swim cap and a coated woven swim cap which had micro holes from the stitching that allowed for breathing.
These days however I found a very controllable, very innocuous, much safer than a bag way of doing it that also eliminates co2 based issues. I just take an N100 mask harness. Pop off the filters, cut off 2 finger tips from a disposable glove, punch a few holes in the tips and put them in the empty filter cartridges. Tada. Not much different then a gas mask, it allows air in with difficulty but is very easy to breath out of, no co2 hazard, very easy to take on and off, strength is easily adjustable