I would also like the Stewart masking screen. Since budget won't allow it, I made my own.
I went to a print shop and bought a 3/4 thick by 4 by 8 sheet of black gator board. It is a foam core with black paper adhered to each side. I ripped four pieces. Two for 185, two for 235.
I crosscut them to fit snuggly inside the frame. The stuuf is so light it will stay. If it did not stay I would have used velcro for the top piece. Works great. I paid $60 for the sheet.
It cuts like wood. I used my table saw to rip, power miter saw to crosscut.
Good luck, Kevin