Researchers conducted successful trials in Bangladesh of a folded sari method for collecting water and filtering. Not only is this a significant improvement in water quality, but it was also easily accepted by the people in the area of the study as well as continuing to be used in a follow up study five years later. This method fulfills the design constraints perfectly because it uses things people in the area already have, since an older cloth is actually more effective than a new one. It could also inspire entrepreneurs in the area to create filters specifically for the vessels used in the area. This is not a long term solution, but one that can be quickly and effectively implemented in the areas that need it most.
See: http://mbio.asm.org/content/1/1/e00034-10 for details on the study.