I just laid 2x4 welded wire across the top, lapping it where needed, and laid a tarp over the whole thing. I anchored the tarp all around, which holds the welded wire in place without lashing pieces together. It's all been in use for about 3 years without a problem; well, almost. Because the tarp isn't drawn taut, puddles formed....large ones....with a lot of rain. Solved that problem with a couple of "T poles" (7'-8' round fence posts, with scrap plywood screwed to the top, set on the floor of the run). The poles are longer than the sides of the run are high, so all the water runs off.