This useful article published in 2013 in Disaster Medicine and Health Preparedness explores the effectiveness of home made masks in protecting against an influenza epidemic. They investigated materials ranging from a cotton T shirt to a vacuum cleaner bag.
Their work suggests that although a surgical mask was 3 times more effective than a home made one, if commercially made masks are unavailable, a home made mask was better than nothing, so long as it doesn’t lull the wearer into a sense of complacency. The best masks were made from a pillow case or a cotton T shirt, with the T shirt being slightly prefered because the stretchy fabric made it more comfortable to wear.
These homemade masks are no substitute for proper medical masks if caring for a Covid-19 patient, but are useful additional protection if you’re somewhere where its hard to keep 2m away from people (eg in a supermarket). They will also stop you accidentally spreading it to other people, (which is easy to do if you are developing Covid-19 because you may not yet have symptoms for several days). However, for a home made mask to be useful, its important to be careful how you take it off so you don’t infect yourself. Masks should be washed after use (ie daily), so you’ll need several.
Download the full 2013 article here
Remember: all the medical advice says that regular hand washing is more important than wearing a mask.