If you search around a bit, you can find some of these:
They are not quite a heavy as what you typically see used for canoeing, but they are cheaper, come in 30 and 60 liter sizes, have hand grab indents rather than handles, and are more squared off at the bottom.
I actually think the design works better ... no protruding handles to hang on things and the more squared off bottom gives you better waist belt contact than a tapered bottom. Also being a little more square, it doesn't roll away as readily as the completely round ones if they are placed on their side.
BTW: 30 liters is about 8 gallons, which makes 60 liters about 16 gallons. Those seem to be the standard harness sizes.
dd