Go to the glass shop or hardware store and get two pieces of glass as worker suggested. It should cost a couple of bucks. If you want to mask, great, but you won't really need to. A little stray light isn't going to reduce the contrast noticeably on a holga (: All that said, try the med format carrier. It might make the edges less focussed, but is that really a big deal here? It may enhance the look you want. The best way to learn about photography isn't asking a bunch of people on the internet the best way to do something, but trying it yourself and seeing what happens......at least that is true for me!