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 (in fact on further inspection the pin will just drift out with a hammer on a smaller pin such as a nail or drill bit). 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. Enhancement: if you can get Bosch HEPA filter cartridges that will take your filtration up to N100 which is 99.97% PM3, far exceeding the N95 specification.

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.

Put the mask on and strike a pose!

Make sure you can breathe easily and check for leaks while still in a safe atmosphere.

Cut out some small squares of material and slivers of duct tape, the front-facing exhale valve is the most important to get covered,

My second attempt on the internal filters was much better, they pop out,  so I stuck a filter on it with cyanoacrylate and trimmed off the excess.

Then popped it back in.. 

You can also  add an extra internal filter at the top which will double-up the intake filtering.

I have adapted the snorkel stalk and added some HEPA filter material, as an alternative to the quick filter addition above.  The stalk is drilled through so that the intake air only goes through the big baggy filter. The exhaust goes out of the top through the "side channels". This "filter cartridge" is more user accessible when it comes to fitting. I chose to do it this way because the HEPA material is much more expensive and also flows air with more effort, so you need to present a decent area to the airflow but at the same time not waste a load of material.