To frame a canvas, I secure the canvas stretcher bar to the frame using a bent pipe clamp. You can buy pipe clamps for very little at a hardware store. There are several types, some made from aluminum, some from copper or steel. I used the light aluminum ones because they are easy to bend.
I beat the clamp flat with a hammer and then re-shape it with my fingers in a way that makes it work for a clamp that secures the frame to the canvas. When the clamp is flattened out, it is about 1,5 inches long x 1/2 inches wide.
The holes are in a perfect location to put one screw in the stretcher and one on the frame. The flattened piece are easily molded to form, around the wood.
I put one or two per side using very short 3/8" screws. It's a very strong method to hold your painting in place within the frame.