I have to admit, I struggled with a similar border that I placed around my own map. It took me far longer to do than it should but I guess much of that was due to changing the size etc.

What I did was calculated from the size of the image how many alternating boxes I would need and then used the fixed size marquee tool to create a grid by stroking the inside with a 1 pixel pencil tip and then moving the marquee. After that I filled the boxes with the paint bucket and cropped out all the lines I didn't need.