On a more serious note, a couple of weeks ago when surgical masks suddenly became unavailable, I pulled one apart and examined the three layers – the water resistant (coloured) layer which goes outside, the moisture absorbent inner layer – and the almost unnoticeable middle layer which combines the two conflicting properties of (a) breathability (b) filtration.
I then went to Spotlight (our local fabric supplier) to look for materials which most resembled the above, with a view to doing some fooling around during the expected period of self-isolation and travel restrictions (which is now upon us).
The nearest I could find for the outer layer, which looked good, was a non-woven microfibre cloth that comes in various weights, I think used for stiffening inner layers, in the garment trade. The one I bought is polyester and I put it under the tap - it is water resistant, though not quite as good as the outer layer of a surgical mask.
I think if I were going to make a Arne-type DIY mask I would put a throw-away replaceable layer of this on the outside, as a first-line defense against droplets.
I think the main purpose of the mask is to lower the likelihood of transmission rather than protect the wearer. (A surgeon doing an appendix operation does not wear a mask because he/she is afraid of catching appendicitis). That being the case, Arne’s mask, with a water resistant outer shield or an improvised celluloid visor (see below), might be as good as any.
The problem is, the high-tech spunbond middle-layer filter in surgical masks does not seem to be readily available, so trying to make a true surgical mask is probably not a DIY task. I bought a pack of vacuum cleaner bags to cut up, thinking this might make the best available inner layer for filtration – but have not yet had time to test “breathability”.
Then googled and saved what seemed to be the best information (and top of my list was the same site as the one on the top of David’s list – recommended reading.)
By far the most practical I could find, on the subject of DIY masks was this one, put out by Hong Kong Consumer Council and including a recommended DIY design from Shenzhen and Hong Kong University Hospital: https://www.consumer.org.hk/ws_en/news/2020/covid-19-diymasks
This is the one I would follow for a simple DIY surgical mask which does not require sewing – augmented by a transparent visor shield made from transparent file-folders which is attached to one’s glasses. The transparent shield and the DIY mask made from household materials, as described in the link above, might be a good option
The New Zealand file folders seem to be a little opaque, but light, stiff cellophane (celluloid?) is readily available by the roll at the local stationery warehouse.
In the meantime, surgical masks are available again in NZ though you have to hunt around for them and take pot luck. I agree with Arne, the high quality dust mask which allows unfiltered exhalation may be a protection for you, but does not protect others and so is not a good choice.
(PS I bought a Hepa filter too, with a view to trying out a "Darth Vader" style, but after looking at Arne's modelling photo's I decided it might frighten the public - though as Arne points out, that does achieve the goal of "social distancing"!)
One more point: Arne's mask, and the cloth type in David's No. 3 recommended website, are presumably intended to be sterilised and re-used. Don't bother tryjng to sterilize a surgical mask, it destroys them. They are throw-aways and nothing wrong with that in these circumstances, as long as they go straight into a lidded bin. Where they are in short supply and have to be re-used, a Chinese friend has told me people leave the surgical masks in direct sunlight when not in use, which may help a little to sterilise, without destroying the middle filter layer.
I'm waiting for some lateral thinking from Howard.