Actually, it might not be necessary to cover it, depending on the personality of your individual rabbit.
My rabbit's cage is 28" tall, and I know that he is capable of jumping that high, but he... idk... chooses not to. He's lived in there for years lol! I'm relatively convinced that if the cage were only a foot high he'd probably never jump out of it either, but it has platforms so it needs to be two grids high.
On the other hand, a flimsy cover might not be enough if he's a super determined rabbit.... in which case it'd be better off if you followed the advice of the others. x)