I used an old rainfly from a tent. It already had bungees and hooks on it, I just hung it from the open rear hatch, and fastened it under the tailgate. I secured the sides onto the hydrolic arms of the hatch with those large spring type paper clips. (Don't know what they are called..they are "modern" style paper clips) No rain issues, or bug issues either.
By the way, the rain runs down the "natural" gutters of the hatch/tailgate when it is open. Even if you don't cover that gap completely, no water gets in. I have been in a really bad rainstorm with it. No problems. The rain drains BELOW the tailgate.
I have a "gooder" replacement tho. Go look in DIY/Mods forum. I just did this tonite. Its a multi duty car cover.