I have been using sheets of mylar cut from a roll which I purchased from the local camera shop... I'm not sure what it is sold for, but it is great as a diffuser. Each sheet has about 1/2 stop reduction, so I have created some "neutral density" filters by taping together 2 and 3 8x10 sheets. So I have a 1/2 stop, 1 stop and 1 1/2 stop filter that I can lay on top of the contact printing frame based on the density of the negaitve. It is easier to make smaller changes in expose thsat way than constantly adjusting the time. The mylar is sold with a name something like Lumilux (?) - - could be wrong about that - - but I'm sure someone here will know better.