Another option is to use a custom BIAB filter bag in the cooler. Even better, add recirculation to pump the wort from the drain back to the top of the grain bed continuously during the mash. Your efficiency will go way up. Example at this link, bottom of the page:
http://www.biab-brewing.com/?page_id=11BIAB or Brew in a bag, is an all grain brewing method that allows one to use less equipment - ideally you mash and boil in the same kettle by lifting out the grain bag before boil. As you get to larger brew volumes, lifting a heavy grain bag can be dangerous and cause injury. So some BIABers have gone to a two vessel system, sometimes using another pot, sometimes using a cooler, to do the mash in, but still use the filter bags in lieu of a false bottom, bazooka screen, or manifold.
I have an electric setup and do temperature mashes, so I have two kettles, both with electric elements, and I mash in one, boil in the other, and use a pump to do the recirculating mash and the wort transfer to the boil kettle.