Use exterior plywood, or marine grade is better.
Other than that, not much to add.
Christopher A. Young
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I'm in the process of getting a new AC unit installed. Reading
older posts here I see where the primary recommendation for those
of us intent on using a cover is to make it simple. Such as a
plywood square on top only held down by bricks.
Primary intent is to keep pine needles and ice icicles away.
Any other recommendations better than the plywood method?