Before you begin, you might want to take a look at this article that shows the best materials to use for masks. Please note, though, that the kitchen towels they mention are not the terry cloth ones, but are tea towels. This is very important. What Are The Best Materials for Making DIY Masks? - Smart Air Filters The Olson Mask is what one of the large hospital groups in my area is asking for specifically because it is a tighter fit than the surgical type and it has a place for a filter to be inserted. Instructions for this mask are here. https://www.unitypoint.org/cedarrapids/filesimages/Coronavirus/Olson Mask with Pattern v4.pdf Most hospitals that are requesting masks are asking that they be two layers of 100% cotton for breathability. I’ve not seen any requests that suggest the use of t-shirt material, probably because it can be difficult to sew. You are better off sticking with the kind of fabric used for quilting or tea towel material. Whatever you use should be tightly woven.