I was thinking of using lith film so it would form a solid block where it blocks light, so it wouldn't really affect local contrast. I was thinking that it would be easier to put my mask on the easel over the paper, because this would be easier to properly align with 35mm film.
Also, (there was a url link here which no longer exists) is the article.
Yes, you can do what you are thinking about doing. For dodging or for burning you may find that it is helpful to make an unsharp mask to blend the demarcations on the print (the unsharp mask would be used after your burn and dodge mask). The Photowarehouse APH material is available in sizes up to 20X24 inches the last time I checked.
The reason that we normally sandwich with the camera negative is the cost of the masking material is less expensive that way. But 35 mm is very difficult to register properly.
You might consider making a fold over fiber foard frame to hold your mask ...that way you can fold it in or out of the projected image as needed.
Another thing that can be accomplished by doing this is to have two different filter grades for your shadows and highlights...So you do not need to expose the entire print at a grade two ...you could print your highlights at grade 1 1/2 (as an example) and your shadows at grade 3 1/2 (as an example).
