I had read about in-line portable water filters a couple of years back, but can't seem to find a source for them currently. They seemed to be just a small housing for a filter with tubing intake and output tubes attached. I would like to make a simple gravity system by modifying the cap of a 2 litre pop bottle to take a nipple fitting that would connect to the standard tubing that leads to the intake of this in-line filter.

Any leads on this product?