I don't know if anyone is still reading this thread, but I got back from a week-long camping/day hiking trip to the Grand Canyon just tonight. I was not backpacking, just day hiking, but I traveled to G.C. without a car and had no hard-sided container for my food. I camped in Mather Campground where there were no ammunition boxes for food storage. On a ranger's (poor) advice I kept all of my food in my tent (a backpacking tent), but on the third day a squirrel tore a hole in the mesh part of my tent, entered, ate a hole in the bottom of my food stuff sack, and tore holes in the two plastic bags in which I had walnuts and crackers. Then he left (in a hurry, I think, when I happened to walk back into camp) via a second hole that he made in a different part of my mesh tent netting. (The second hole was at the apex of the tent, but there was no damage to the sides of the tent. He must have taken a panicked, flying leap upward to escape…)
I temporarily patched the holes in the mesh with some sticky tape I had for emergency air mattress repairs. I couldn't carry all of my food with me when I left my tent, so I put my least odorous food (freeze-dried dinners and instant oatmeal) inside a canvas duffel bag inside my tent, and I placed the rest of my food (gorp, cheese, nuts, dried fruit, M&Ms, etc.) inside my daypack which I kept with me. I then left the tent and went to the General Store to see what kind of a solution I could find (short of buying a ratsack for $49 or a cooler for the same amount). I found a large heavy plastic drink jug with a plastic screw-on top – it said "Gatorade" on the side – for $12 and bought it. I also bought some duct tape.
Back at the campsite I found that the squirrel had reentered my tent and chewed two small holes in my canvas duffel, but he hadn't gotten all the way into it, and I had not lost any more food. I didn't, however, want either my tent or my duffel bag totally destroyed… so I took the food from the duffel bag and stuffed it all in the plastic drink jug, screwed the cap on, and put a huge rock on top of it. (The rock was to deter ravens. Ravens were systematically tearing up campsites all around me whenever anyone had left any food out.) I could not get ALL of my food in the drink jug and continued to carry half of it around with me in my day pack for the remaining four days that I was camped at Mather.
My jug-plus-heavy rock remained undisturbed for the duration, and the squirrel didn't go back into my tent for another three days. I left no food in my tent at all now. But on the day before I left G.C. – the sixth day after my arrival and the third day after I'd bought the jug – the squirrel went back into my tent and chewed a hole in the plastic bag in which I had clothespins and a clothesline. I have no explanation for this other than the squirrel's prior habituation to my tent.
My total losses were some walnuts, some crackers, a couple of little holes in my duffel bag, and two larger holes in the mesh of my tent…
Ravens never bothered my tent. I concluded that ravens find food by sight, while squirrels find it by smell.
I don't know if a rat pack would have been good enough to keep that squirrel out. If I return to the Grand Canyon I will use something like that drink jug, or even a bear barrel. And I will scrupulously keep all food out of my tent.