I believe I got this idea from here a couple of years ago. It's just a tailpiece and a couple of clamps. I put wing nuts on it so it can be moved around by hand. There is a little rubber washer between the two clamps. The whole thing weighs almost nothing, and it's cheap to make.
The size of clamps is visible in the second picture. As you can see, they are offset slightly so if your rod was on the left side of the canoe, attached to a thwart, it would be angled forward slightly.