Target your merged sky layer and draw a square marquee extending just outside the circle area. Use your extra channel as a guide (by double-clicking the Channel icon you’ll be given the option of adjusting the opacity, which will make things easier). Expand or contract the selection as required. When you’re happy with your selection, copy it to the clipboard.
Create a new document using the clipboard as the preset. Ensure that the background content is set to transparent and paste in your selection. Now comes the fun part: select Filter > Distort > Polar Coordinates and select the option Polar to Rectangular. This much under-used filter converts a selection from its rectangular to polar coordinates, and vice versa, according to the selected option.
Choose canvas size and uncheck the Relative option. Set the height to 200 per cent using the drop-down menu and anchor the image placement to top middle. Duplicate the layer, rotate by 180 degrees and flip it horizontally. Now hold down shift and drag to the bottom of the canvas, leaving a pixel or so of transparent canvas at the bottom.