Most manufacturers today use finishes on their cabinets that withstand the
elements and won't rust (I said MOST). One thing I have noticed is that the
units that customers DO put covers on will stay looking new longer than the
ones that do not. Yes, covers WILL hold in moisture and may cause you damage
to the electrical components in the unit. The best cover would be one that
allows air to flow through, so you would want one that maybe has a 6 inch
opening at the bottom. I've seen some covers people paid WAY too much money
for because they were supposedly "custom fitted" for their particular unit.
A small tarp and bungy cors work just as well.

Not very pretty, but the best one I have seen was a piece of plywood with a
brick on it. Simple, yes. But it kept the leaves and debris out of the unit.

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Both my sons are HVAC types...and when I purchased a cover a few years
ago for my a/c unit BOTH gave me holy hell...

I listened to them...and
Now use the 2x2 piece of plywood & a couple of bricks...

Bob G.