This technique works pretty well, allowing for a bit of evaporation, but keeping most of the moisture in. At the cooking school where I assist, we show our students how to make these parchment circles, using a large square piece of parchment paper, folding it over and then over again to make a small square, then folding it over again twice the way you'd make a paper airplane. Once you've done this, you cut or tear the resulting folded piece so that it is exactly the length of the radius of the pot -- and when you unfold it, you have a perfect circle, which you can either pierce once in the center, or leave intact. It's pretty quick and easy.
Actually, there is a pictorial demo of this technique at: