Yet another use for a poncho. A space blanket would work too. Some bag style space blanket bivies may be large enough to work over the foot of your sleeping bag. You don't need to pull it all the way up.
If you use a trash compactor bag as a pack liner, you could use that on the foot end of your sleeping bag, or just use your pack.
I use a bivy now, but I used to carry a 45 gallon LDPE trash can liner for emergency use. It will cover to my armpits as a half-bivy. They can be split open to be used as a ground cloth, used overhead to waterproof a makeshift brush shelter, as a windbreak, or even a solar still. Cut holes for head and arms and you have rain gear. LDPE is light, but contractors plastic trash bags have all the same uses at a bit more weight.