I'm guessing Bob is going with the "belt and suspenders" approach by using two layers of foil. The sling is the thing, though...suspending the foil off the inside of the pan just a bit, leaving an air gap that insulates to minimize burning fat.
Grease almost always finds a way past the foil, through a pinhole in the foil or by salt corroding the foil and making its own hole or through the seam where you connect two pieces of foil to make it wide enough to fit. In my experience, any grease that gets past the foil seems to clean up pretty easy, it's when you have a pool of uncovered grease that you let bake down to nothing that you end up with trouble, and it sounds like that's what you've got.
Use a plastic or wooden implement to scrape out what you can. Soak in hot, soapy water several times and keep scraping. Eventually you can apply oven cleaner a couple of times. That and elbow grease and a non-scratch scrubber like a Dobie or a blue Scotch-Brite Non-Scratch Scrubber (NOT the green one) and you'll get it.
Good luck!