Here's a rather ingenious screen-in-a-trashcan that someone just built. This type of design will eliminate evaporation (and cooling), if that's what you want. One thing I might change would be the distance of the bulb to the screen; it should be so close that it almost touches it. In all the builds I've seen so far, the ones that have slow growth always have the bulb too far away, or too small a wattage.