I do it all the time. The weights range in size from boat holding to decoy long lines. The boat holding ones, the concrete is poured in plastic pots that trees are planted in, about three gal. The decoy long lines are held by one quart milk bottles with the top cut off. I also used one gal milk bottles cut in half. I leave the plastic on. I buy eye bolts at the hardware store with a nut and washer. You cut the tops off from the top of the bottle and juggle the eye bolt in. The eye bolts are about three inches long.
I have used round bottles but square ones store better. You can use ready mix but I would add more cement and sand. Makes a smoother anchor if the plastic comes off. I don't use them on individual decoys but my partner Mark does on a few. They have a wire loop that slips over the decoys head.
You can make a lot of anchors for very little cost. I prefer plastic over metal cans no rust and plastic doesn't scratch paint.