Mirror edges for stretch frame canvas print.
Make a new bottom layer and enlarge the canvas by one inch on each side.
I made a one inch selection of each side of the image and copied each to its own layer. Then use Transform>flip to make the mirror image. Then move it out into the empty border area.
Depending upon how the stretch canvas is cut, the new sides may be left rectangular with no image in the corners, or they can be stretched to fill the white corners to allow overlap tolerances for mounting.

The guides are just to illustrate the edges of the original image.

forum discussion: http://forums.dpreview.com/forums/read.asp?forum=1006&message=29233990