A drift sock will pull you along with the current. Not what you're looking for. Try pulling a magnum bottom bouncer of sorts. An anchor without tinges. The appropriate weight depends of course on depth, current, and desired drift speed. I'd try to find an old (window) sash weight and tie a light anchor line (parachute cord comes to mind) drop it to the river bottom. The right amount of weight. once determined, will drag SLOWLY along the bottom and being a slender profile should rarely hang up. May be even a length of heavy gauge chain. guessing 10-20 lbs worth. This too is dependent on bottom composition and other river content, i.e. boulders, trees, abandoned autos and misc debris. Sash weights can be found at garage sales etc. for a cheap price and in the odd event of a hang up, be cast off to be quickly replaced by another.
CAUTION should be taken though when, we'll call it, anchoring/dragging in moving water. Especially in a canoe. Tie off must be on the canoe's center line. ALWAYS use a slip knot. ALWAYS have a dedicated knife tethered along side the tie off in the event of a difficult hangup and your slip knot fails to slip, the cord be cut ASAP.
