The design you followed for your mock build is the original way of building. Many rmh's are built that way , including mine. With the proper dimensions it will work as you have it.
Current designs are using ceramic fiber board and ceramic fiber blanket as a core and riser.
By reducing mass at the core, full rmh temperatures are reached very quickly , this allows for the 98% efficiency / combustion to begin sooner than it will if you have to heat your core/riser firebricks.