I googled around and there are mask patterns everywhere. I found one I liked with a nifty filter area but it wasn't comfortable. I found another but it had no filter. So I took the two ideas and I put them together to make a comfy mask that also has an opening type pocket for placing a filter. Here is the link to the free pattern that I used, it is available at Eat Pray Create.
The filter pocket part is confusing and you may want to skip that and just make it with no filter pocket.
Also, I tried this with heavy material and discovered that using lighter weight material works better, so I used a t-shirt and a bandana.