Man I grew up with the old pikau pack, we used an old 25kg Salt sack, a couple of stones and some 10mm rope I spliced up especially for it.
They held lunch, a swandri and a dozen possum skins and traps easy.
And it's already been said, the best thing about pikau's is their compactness.
I use a Ridgeline Pikau for (mainly) fishing. It holds everything I need for a day on the river and is quite comfortable if packed right. The trick is to make a pad of your raincoat/jersey/ground sheet and put that against your back. Mine came with a sqeeze slider to lock the neck rope off, but that quickly proved ineffective (junk on the ground!:-P), now I pull it tight and tie a slip loop next to it to stop it sliding open. So far so good. Once when my ride was late and I decided to leg it, I managed to get hip waders in as well as the rest of my day junk. I had to tie a loop of cord from bottom to top to hold it all together so I could tie the neck up, but it worked!
Only thing I would change would be to make the bottom more sturdy, a layer of canvas inside to form a shallow 'bin' would be perfect. It would be an interesting and useful project to build.
For a bigger one, a cut down grain sack and some old car seat belts work a treat:-)