So here are the plans for my Noborigama which features an extended firebox and step-grate system. I am expecting the extended firebox to act similar to a train kiln, depositing a lot of ash on the pottery and other clay work, while the chamber behind the firebox should have minimal ash. It will be constructed out of large, industrial, rotary kiln brick and standard high duty firebrick.
After reading Dick Lehman's article "An Approach To Long Woodfire" I was sure that I wanted to build in the "step-grate" system he mentioned. The nearly four feet of stepping grate (which has mouse holes coming underneath it... maybe gopher holes would be more appropriate in this case) should allow me the option of stuffing the firebox with wood and very slowly firing accumulating ash for a few days before really kicking off the firing.
The last picture shows the kiln with its side cut off to better show the step grate... I may even add one more row of brick to the length of the firebox, extending the top step. This would add another 8" roughly to the firebox.
The design needs a little bit of tweeking and dimension checking, but is nearly ready to go.