https://imgur.com/NAXKrrJ
at the middle of these fence board, I am about to install a 2x4 x 8ft at the very top (and at the bottom), and I highlight it in black at the photo. As I will be putting new fence boards just like the way it is at the left & right of the photo.
So that will create a gap BEHIND that 2x4 x 8 ft. I want to fill up that gap, level it, I need a space filler. Now supposedly, if I install a 2nd 2x4 x 8ft, that would somewhat do it, but w/ a 4 mm gap. I want the top totally fill up
What are the options? what can I do?