Regarding the water, I use tap water treated with a chlorine remover and left sitting overnight.
Regarding the screen top, I make them out of screen door materials that you can buy at Lowes or Home Depot. It is very easy, you just buy a strip of the aluminum channeling, plastic corners to connect the channel, a roll of aluminum screen, and the tubing to lock the screen into the channel. There is also a small roller tool that helps with inserting the tubing into the channel. All the materials are in the same place by the screen doors in the store. You need a hack saw to cut the channel and a file or grinder to smooth the edges. I make them so they fit very tight inside the aquarium rim and I add a knob to the edge for easy removal. Then you can either use plexiglass or a piece of real glass cut to fit the inside of the screen cover. It's easy to control humidity if you use two smaller pieces of glass and vary your opening for the amount of air you need to let in. The end result is some very clean looking, tight fitting covers that are easy to make and repair.
Hope this helps - Dave