I picked up a 3"X 2' solid aluminum extension and the eight foot aluminum flex pipe for cold smoking. I have a hatch in my deck up against my house. I have a large Kamado with an additional elevated rack and a removable bottom ash pan. I'll probably steal your idea I've seen in your pics and pipe it straight up from below from the mailbox.
Kurt, you will find that single wall stove pipe will dissipate heat much better than the flex. You are doing fine, now some fine tuning. Put enough slope on the run to the foodbox so the condensed liquids will flow toward the firebox.