There's an "how to" for that kind of backpack somewhere on the internet.
I made one from the ground sheet of a cheap old tent, similar material, and use a therm-a-rest (the foldable foam one) as back-frame replacement. With all gear (including tent, sleeping bag, mat, camping stove/pots, water and snacks) I'm down to about 8kgs now, using that pack. And that's for a winter setup! The pack cost me 5 Euros alltogether (and a few hours of work).
Not long-term-tested yet, though. I'll tell you about the results as soon as I know!
edit: with regard to the liters - not sure. My huge winter sleeping bag, all clothes, cooking gear ect. fit in easily.