I've recently started using a day pack that can be stuffed into its own lid pocket that may fit the bill for you. It's light & takes up very little space in a larger pack or suitcase and then can be un-stuffed when you are ready to use it. The one I have is a MEC Travel Light Top Loader, but that's a Canadian source.

Sea to Summit has something similar which you may want to look at, the Ultra-Sil Day Pack.
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Victory awaits him who has everything in order — luck, people call it. Defeat is certain for him who has neglected to take the necessary precautions in time; this is called bad luck. Roald Amundsen