I have been cutting the pleated face masks 9 x 6.5 inches or 9.5 x 7.5 inches.
For the smaller ones, I fused a piece of 5 x 5.5 pellon 911ff to the center of the outer layer.
For the larger ones, I fused a piece of 5.5 x 6 ti 6.5 pellon 911ff to the center of the outer layer.
When I ran out of that, I used a non-woven interfacing cut the same sizes and then sewed the pieces to the outer layer.
As an experiment, I laid out various combinations of fabric - interfaced and not - on paper - and then sprayed them with a mister.
The area where the fabric was interfaced - definitely helped keep the paper dry for some time.