I'll investigate manufacturing a window screen to try, I live in the UK and this is not something that's readily available. (At least I've never seen them.)
Frank, I've been using a similar product just called "Print Flattener" for a couple of years now and had good results with it. I'll have to check when I get home but I believe it's made by Edwal. It doesn't leave the prints "perfectly" flat, but I just place them under a few heavy books after to completely flaten double weight paper. It does take most of the curl out of the paper with no side effects that I've seen in at least the last 2 years.
The easiest way to flatten a print after it is dried, barring a dry mount press is to iron the thing. just use two pieces of heavy paper top and bottom to protect it, and iron away at a lower setting. It will not harm the emulsion in the least.
You're absolutely correct Aggie. Dry mount presses are expensive. If you're just a home darkroom worker, you won't necessarily have enough need for one to justify such a purpose. Cheap print dryers, the kind with the canvas covers don't do such a good job and aren't worth the money either.