Hi Fox; Welcome to Permies!
Well, Fox the cast or built J tubes have about reached their potential, with the peter channel and the trip wire. No longer are folks casting but using ceramic fiber boards instead, to build a core and ceramic fiber blanket to build a riser. They are the newest thing in J tubes.
All the newest experimenting has been with horizontal batch boxes. Burn time is only marginally extended with a
BB, but the temperatures are noticeably higher ! There is no open flame with a batch box.
By all means experiment with J tubes, innovation and radical free thinking is what developed them in the first place, batch box's came about when j tubes reached a temperature max and the innovators wanted more... See if you can improve on the J tube design and surprise us all !