All, be wary of using foamboard for use outside. It may warp on you. I have a piece of black foamboard I use as a light block on the back of my 10" Newtonian. It has warped over time due to heat of the day and the dewing at night. Foamboard is plastic foam with paper coverings. The paper and adhesive are the culprits here, especially if one side is more to the weather than the other. The curves that form are permanent. As with my light block, creases were formed when I readjusted it to fit the back of the scope. No problem there, but may be a concern with this kind of mask.
Joe.