Now this all depends on how much white background you have available, you don't need a lot but a couple of centimeters may be a bit small.
First step is to enlarge the canvas. You wont need a lot more.
Now use the rectangular selection tool to select the background between the subject and the Lastolite edge. Now go to Free Transform. Grab the appropriate handle and drag the the appropriate part of the selection to extend it outwards. Now crop to exclude the Lastolite edge.
Technique works well at extending skies as well!