I think what you are looking for is what some call the toilet seat cover. A cloth gasket cut into a donut shape to fit between your mask and your face. Similiar to those paper covers you find in restrooms. It has been covered in a number of posts, but to make one it is pretty simple. Lay the cloth over your mask, cut around the outside edges (leave about a 1/2 inch border. Then cut a hole in the center the size of the inner opening of your mask.
You can either place it on your face before you strap up or as an alternative when cutting the outside leave some extra coth strips to attach it to the outside of your mask.