As far as your metal cover goes, can you find/make one out of plastic? And do you even need one?  
Guppies aren't known for their jumping ability.

As far as evaporation goes, it's winter here too, and I can see the effect it's having on my tanks as well.

I also have hard water in my area and an extemely high pH.

I read somewhere that a mistake that is often made by aquarists is that they only refill the evaporated water, and don't do a water change as well.

What this does is actually increase the water's hardness by concentrating the minerals already present, and adding more with the "fresh water".

Something that I found to be true, when I was trying to combat increasing hardness levels in my tanks.

What I do now is, when I see that a tank needs to be "topped off", due to evaporation, I clean the exposed glass of minerals,(sometimes you can see a residue) and syphon out an equal amount of missing water.
So now you are adding back twice the amount of fresh water than what has evaporated.

I have noticed a significant improvement in the tests using this procedure.
Another "guppy gal" that was having a similar problem adopted this method, and is very happy with the outcome as well.

I don't know of a way to remove lyme from water other than by filteration.  Another idea, though it may be expensive is to use distilled, or bottled water for your tanks.

I still recommend changing water out whenever you are fighting evaporation.