Why not just use a test print to make a mask? Use an Exacto knife to cut out the areas that need to be burned some more. Hold that over your print on the easel when you go to burn in the light areas. Give it a little jiggle so the edges blur.
If your paper isn't opaque enough use it as a stencil to cut holes in a piece of thin cardboard.
I wouldn't do it with Photoshop. Not only would you have to wade through all the Photoshop BS, it costs you ink and transparency material.
You've already got a test print that you're probably going to toss out anyway. For the cost of an Exacto knife, which you probably already own, and a few minutes worth of handy work, you can have a mask in about 5 minutes.
