To me, a coil cover is a waste of money and sometimes more trouble than it's worth.
If you decide to get one, put marine silicone around the edge to seal it. I once had one that would accumulate water when swinging early in the morning while the dew was still on the grass. Not much, mind you, but enough to drive my detector nuts.
I've put a few miles on coils in some fairly rough terrain and haven't worn one out yet.