However, this is contrary to the fasting process. We need to refrain from chewing and stimulating the digestive process.
I have had good luck packing juices in bags, air tight. Put the bags in a leakproof container (like a yogurt container) for support. Keep very cold and they do well up to two days.
Beyond two days... if I were camping I would take oranges and a small hand juicer and stick to oranges til I could get back to civilization!