Way back in August 2014 I decided to modify my Sawyer filter bag to make it easier to fill because I found it a bit of a struggle to fill.

So I ordered a couple of 2 litre bags and proceeded to cut the top off.

Then I cut a length of plastic plumber’s pipe just wider than the bag and split this down one side,I also cut a piece of old hydration tube a bit longer than the plastic tube.

I filled the bag with water and then folded the end of the bag over the hydration tube,then slid the plastic pipe over the bag and tube.
As you can see in the above photo it is a really strong seal.

Field testing the bag it was easy to fill the bag one handed.

I have also added some string initially to hang the bag from to enable gravity to filter my water (lazy) but in the field I found it very useful for carrying the water to a suitable place for brekkie.(about a mile with no slippage or spillage).
In conclusion one of my bird brained ideas actually worked: -)

Update 2018

I have updated my initial modification with better quality plastic tubing (available from b&q) & instead of cutting the bottom of the bag off I now cut the inside of the bottom of the bag off leaving more room for folding without reducing the volume of the bag, the new & improved design now weighs 60g (20g less than a specifically designed version)