The lines of the graticule are 15° apart so the outer boarder is just in evenly spaced 3° sections. At the equator, I cheated and just divided each graticule spacing into 5 equal sized boxes. This is not strictly accurate but close enough for this map. Inkscape would probably be a better tool for this sort of precise repeated line work thing than Photoshop. As for QGIS, I think you might want to hold off on that for now.