Here's a way to secure a big canvas on oversized stretchers into a frame. It is hard to stock offsets in every size and so I fit the big canvases using screw eyes. First I put the canvas into the frame, and with a pencil I draw a line one the side of the stretchers just above the rabbet, like this.
Then with an awl I put holes along that line, several on each side and several on each stretcher side of the canvas .
Then I remove the canvas from the frame and put a screw eye in each of the holes I have made. I turn the screw eyes into the wood using my awl for leverage like so....
Then, after I have placed all of my screw eyes, I put the canvas back in the frame. The screw eyes should sit just on or slightly above the frame, like this.....
Then I put a screw through each of the screw eyes, into the back of the frame.
Here's what each screw eye should look like when it is finished.
Two or three of those on each side of the canvas should hold the painting in its frame. It also looks real slick.