I found one of these on ebay - a used military kit bag which can be carried as a backpack.
This one is from here
- no affiliation, just the best photo I could find.
Mine is huge, light, incredibly tough, and I'm not scared load it up as much as I feel up to carrying. Mostly I use it for mulch or collecting pine cones for kindling. It's also done service for laundry, travel, carting tools around, all kinds of things. It has a strange fastening system which means that if you don't fasten it, you can carry longer items in it and just leave them poking out the top.