Some trunk bags are insulated and mostly waterproof, so they can be used as a makeshift cooler. Fill with ice and your choice of beverage once at camp, and maybe it'll also work for perishables overnight.

I adapted a $3.99 6-can soft cooler/lunch box for use as a handlebar bag, and it does keep the inside dry in the rain, except for some minor seepage through the zipper in a downpour. Much better than a straight nylon bag I had before, and I do use it as a cooler once in camp, too. The best method is to stop at a dollar store before camp and get a small Styrofoam cooler, but that's not going to be feasible everywhere...I tried to leave the cooler at the hike and bike site for the next day's travelers, but the rangers insisted on scouring the campsite clean every morning.