I finished this gravity filter setup recently and I'm not enthralled with the result, but I thought I'd post photos and a description of it here anyway. The whole system (including water bag, carabiner and hanging line, tubing and fittings, and filter) weighs 3.65 oz.
The bag is a silnylon rolltop stuff sack with a 9mm kitesurfing valve installed in the bottom and sealed with silicone adhesive. The result isn't pretty, but it is lightweight and watertight.
When I fill the bag and carry it back to camp, I don't want the filter and hose attached because I don't like having to worry about the dangling tubing or about keeping the clean end of the filter clean. So, for this setup, I included a short length of tubing with a valve that has no clean end to worry about. The long (5') bit of tubing attaches to this once the bag is hung.
On the upstream end of this little bit of tubing is 3" of rigid polypropylene (drinking straw-like) tubing that protrudes into the water bag. I've found that this almost eliminates the problem of detritus (leaf/conifer needle bits, silt/sand, twigs, etc.) getting into the tube. That stuff settles to the bottom given about five minutes undisturbed, and the water that goes to the filter is much cleaner. I'm also considering trying alum flocculation to remove tannins and organic particles, and the protruding tube would keep the settled floc from entering the stream of water moving toward the filter.
On the downstream end of the short bit of tubing (below the valve) is a fitting that permits the long piece of tubing to be attached. I couldn't find the kind of fitting I wanted, so I had to make this myself. It is a threaded port from a Fresenius hemodialysis filter, joined with epoxy to a nylon barb, and then packaged in a carbon fiber tube that I layed up with 3 layers of 6K 4H satin carbon cloth and Aeropoxy. On the end of the long tube that goes down to the filter is a nylon DIN adaptor (also for hemodialysis) made by a company called Molded Products. This screws into the upper adaptor once the filled water bag is hung up.
The DIN adaptor is joined to the 5' tube that goes down to the filter. That tube is made from thin walled lay-flat (collapsible) polypropylene tubing with a black braided polyester sleeve. Because the filter is the flow-rate limiter, and it is at the bottom of the tube, hydrostatic pressure keeps the tube open ("inflated"), so it doesn't flatten or kink. The braided sleeve protects the thin walled inner tube from abrasion and punctures. I chose this combination because it is about 1/2 the weight of conventional gravity filter tubing, and it can lay flat and be rolled up into a very tidy and compact little "puck".
At the bottom end of the long tubing is a water bottle cap. This attaches to the filter itself. I designed it this way so the prefilter could be accessed easily (for cleaning or replacement) and so the upstream end of the filter could be uncapped for backflushing.
The filter itself is made from the neck of a polyester water bottle, a Platypus bottle cap, two carbon fiber hoops, and the innards of a Sawyer 0.1 micron inline filter that I received as a gift but never used. The hollow polysulfone fibers of the Sawyer filter are visible through the (polycarbonate?) tube that encloses them within the Sawyer inline filter housing. I like this because I can see how clear or turbid the water is as it reaches the filter.
There are two prefilters that can be removed for cleaning or replacement. The first is just a felt disc from an old Frontier Pro and the second is a 5 micron filter disc used for prefiltration of reagent water in biology labs.
At the bottom of the filter is an outlet hole and the Platypus cap, and it screws directly onto the clean-water bottle.
For packing, I leave the filter and the 5' hose attached.
The flow rate of this setup is fine for my purposes. A one liter Platy bottle fills in 33 seconds. I would be happy, actually, with a much slower filtration rate, and I might use a smaller filter if I make another one of these.
Backflushing in the field is easy. I just remove the cap and squeeze the clean water bottle. Incidentally, a "tornado tube" bottle connector could easily be used to make this into a modified Sawyer Squeeze, but I haven't tried that yet.
Sorry for the long post and the glut of photos. Any feedback is welcome.