Area above the J-tube

Hey folks

I am needed to build a new rocket mass heater. I am moving my tiny house and really think its best to disassemble the previous stove I built a few years ago.
Due to time and energy levels... I am wanting to build something simple without requiring custom parts. Along with using stuff I already have the supplies for. The previous stove was a custom made one based off of Matt Walkers's riserless core. Link to my previous stove https://permies.com/t/tiny-house-masonry-heater

What I have in mind this time, is just a simple 6" J tube, where the feed is outside of the brick bell, while the riser is inside the brick bell. Right now I picture it being a rectangle in size, with one corner cut off for the j tube feed. About 48"(W) x 32"(L)

Anyways my question is about the area above the riser. I am thinking about a 16" feed. So the riser needs to be 48". So I think I need part of the bell to be atleast what...... 12" above the riser? Or close to 5 feet tall? I was trying to find this answer and quite frankly don't have the time to read every thread on here for it.

I am thinking of a flat top, and single skin.

Something like this

Hey Jordan;
A Batchbox requires a 12" clearance.
A J-Tube can be much less.
A minimum of your chimney diameter, so 6-8" would work fine.

The beauty of this style build is that you have the option to "upgrade" to a batchbox later.