I don't mean to sound ignorant but why are you covering the wells? If you were to leave them uncovered, does the rain and/or melted-snow present a water problem? If not, then I'd leave them uncovered. (See below on an idea of how to keep the well 'clean' if you decide NOT to cover the well completely.)
FWIW, as an aside, I have a 30' x 15' crawlspace under an addition. One of the vents happened to be about 1" above ground level! This was bad when it rained heavily or snowed. Water would run in when it rained, and show would block the airflow. So, I dug deep enough to put in a half-moon well, then graded the bottom away from the footing, then filled it with some gravel to about 8" from the bottom of the vent. I then layed a half-moon-sheet of filter fabric (you can use landscape fabric for this) on top of the stone, and put a final 2" of gravel over that (to cover the fabric so you wouldn't see it). By doing this with the fabric, when the well fills with leaves and stuff (mine is uncoverd because of the venting), I can lift out the fabric, stone and all, clean it, and replace it. (If you let the well get 'contaminated' over a number of years, even with the fabric, the junk can impede or prevent water from filtering down, and therefore, end up in your basement. The fabric prevents any junk from getting any deeper then 2".) Since I can't put a bubble over this particular well, I actually built a small roof over it. I used 3/4" PVC piping for the 'stand' that holds up the roof, shingled it, and pushed it into the ground to about 12" above the rim of the well-surround. Ta-daa! A window well with a roof. It 'breathes' yet it keeps out the 'elements'.
If you have moisture in the well(s) *AND* you know the only cause for it is from whatever evaporating rain (or melting snow) is trying to get out, then leave a small gap where the cover meets the house. Yes, some moisture from above will get down there unless, of course, you're inadvertently promoting an inward flow of water. There shouldn't be much moisture trying to get out. When the sun heats the plastic-covered well, the moisture condenses on the underside of the cover, in a desperate attempt to get out. By leaving this gap, you allow some air to get in and some moisture to get out. I mean if there's standing water in the well, you have a completely different problem! Talk to me more if you have more to offer.