Every now and then I buy me a 5 liter pack of extremely delicious æblemost (apple juice) from Samsø/Denmark. Now the 5 liter boxes have a disadvantage: the tap lets juice out but no air in (//edit: that's not the disadvantage). So it's tricky to get all of the juice out of the plastic skin inside the box (that's the disadvantage). For the last one and a half liter or so you have to lift the back side of the box so that the rest of the juice can flow to the tap.
This is what this "Saftschlitten" (my brother's coinage) is for. And a picture paints a thousand words...
Print two of them mirrored along the x or y axis. Printer settings aren't that much strict. It's what I used to print it.