If you look at post 205 in this thread I have described how I added extra space to the image using Perfect Resize. I have now done 2 more of these where I have added 3.5 cm to each side ie an extra 7cm to height and width so far I have no complaints at all.
This is how your image would look with the canvas wrap added the reflected edge is what would be wrapped around the canvas so you should not lose any of your image.
If you do not have access to Perfect Resize you could try cloning the extra width and height.
I cannot of course guarantee that adding 3.5cm as I have done will be perfect every time but so far it has worked ok for me.