I have the same pen you do and it won't take long for that grass to turn to all mud. We took three or four 2x4x10's and cut knotches on the edges so they would fit on the top from side rail to side rail. I used chicken wire to cover the entire top and down a little on the sides (just a couple inches) then bought a tarp to go over the whole top. Used zip ties to fasten it. Straw covers the ground for their bedding and it's working out great. The straw stays drier and I don't have to replace it every time it rains now.
