In my experience, none of the automated "make seamless" scripts work very well. Try this (I just had to do something very similar yesterday):

1. Create a plain white layer, and then hide it or whatever. Call it "DisplaceMap".
2. Filters -> Map -> Displace. For the x and y values, use the dimensions of your image, and next to each, set the map layer to your DisplaceMap (the plain white layer).
3. Grab the clone stamp tool, ctrl-click a spot away from the seams that you should now see, and then paint over the seams with it.

Repeat steps 2 and 3 until after the displace you can't see seams.