A window screen, homemade mask is the best:
1. It doesn't block fresh air intake;
2. It doesn't store up bacteria and CO2 on the inside... to be breathed back in, thereby weakening the wearer;
3. The idea of the mask being worn increases placebo effect, so both the wearer, and others are safer, because they FEEL safer... placebo effect;
4. It allows herd immunity to get around faster, thereby making the whole community safer;
5. It gives a good laugh to those who won't wear a mask at all... laughter is a cure in itself.