I've been down with flu, and ready to get back to build.
Here is what I'm considering. A 6" system, with 12" feed tube, 18" burn chamber with a 42" Riser. The riser is offset 2" from burn chamber and my thinking is it can help improve efficiency. This also causes the chamber to 6 x 4.5" causing acceleration (I hope).
Help my diminished mental capacity to achieve goal, or are you thinking oh crap, this guy is going to burn down shop.
Peter van den Berg and other researchers have tried an offset riser and similar configurations, and found it didn't give any advantage. I would advise building your first J-tube by the book; then when you experiment later, you will have a basis for comparison. You can't know if something is better if you haven't seen the thing you're comparing it to.
The Greenhouse of the Future ebook by Francis Gendron