I was using drywall screwed and spackled onto a wooden frame. But I was unhappy with the results, VERY hard to get it completely flat and without any bevels (which are put into every piece of drywall on the left and right to accomodate for screws and tape).
So I tore the drywall off the wooden frame, bought some blackout cloth for $25, stretched it on, stapled it and am VERY happy with the results. No i'm going to paint it with Ddog's formula -- which -- if you haven't investigated, do so. It's the best DIY screen formula available. Even better than 'goo.'
So my recommendation is: make yourself a wooden frame out of 1x4's (probalby around $30-$40 total) and get some blackout cloth (4 yards for about $25) and staple it on ($20 for a staple gun with staples). Then paint the Ddog mixture on it (which will cost around $50 total)
Total for a screen that's better than FireHawk and goo -- about $140