Inspired by the snorkel mask repurposed for a ventilator, I added filter material instead to make some serious Personal Protection Equipment
I went straight onto eBay and ordered a few. Right now order some separately from different sources or if from the same source wait till they post the first one then order another, to maximise the chances of one arriving, as deliveries are getting quite sporadic with the coronavirus disrupting everything. Try and get the type with the separate plug-in snorkel. If you get the type with the fold-down snorkel then you might have to saw it in half, drill out the hinge pin, or put up with the filter suck right on the end. The plug-in ones will be easier to use as proper snorkel masks after all this is over!
I got hold of a car pollen filter. You could even steal one
out of your own car! Cut the material out of the cartridge using a craft knife. Cut out a rectangle of material.
Fold the material over the snorkel mounting post, making sure ALL the edges are more or less level, and fit the first tie wrap "upwind" of the edges so no air can get in unfiltered. Once you are happy fit the second tie wrap above the first. If you use re-usable tie wrap with the tang, then you can re-use them when changing the filter material. Make sure the tie wraps are nice and tight.