I use a large about 2 1/2 inch ID nut as in nut and bolt. I bought at a surplus store, weighs enough to hold the canoe and cost almost nothing.
I constantly see things that could be used to anchor a canoe at goodwill. small dumbbell weights, window sash weights old ankle weights milk jugs full of sand work, look around you will find something that looks good and works well.
A search of canoe anchors on the web shows weights from 1.5 to three pounds. a four to five pound weight ought to hold you in any conditions you should be out in a canoe in.
Eric
"Complexity is easy; Simplicity is difficult."
Georgy Shragin
Designer of ppsh41 sub machine gun