I went with 2 parts fireclay: 1 part sand: 1 part medium grog. I also added some sifted wood ash, hair clippings (from the barber), and dried grass.
I'm happy to report I finally got this thing built enough to test fire it just yesterday!
The J tube was easy to light and had good draw right off the bat. To my surprise, the heat was drawing up through the clay pot, but it wasn't extremely hot coming out. I was able to stick my hand inside the oven without burning myself. The barrel of course got extremely hot right away, so once I insulate it the oven should be much hotter. Using just the J tube, the oven seemed to hover around 200 -250 F. So using the J tube alone, I think this would make an excellent smoker. It also holds the heat for quite a while after the fire goes out.
Today I decided to try lighting a fire inside the clay pot to see if I could achieve higher temps. I discovered that, with the J tube firing, any fire or coal inside the pot seemed to smolder and not stay lit. This is because the gasses have no oxygen once they've gone through the J. So instead I started a fire inside the clay pot without lighting the J tube. This worked great! The J tube unlit just acted like an air intake. The fire got raging, the temps reached 650 very quickly, eventually settling to about 400 and stayed their for quite a while. The oven maintained heat for a long time, slowly dropping, once I put the cap on top of the lid. Of course, this method is not clean like a rocket! There is smoke.
So it seems these are the 2 ways of using this thing. I think this design has many advantages. One nice thing is there's not much cutting of metal, only a few basic cuts that are pretty easy. Also, I cleaned it out after the first couple firings and it was so easy! The shop-vac tube goes right into the hole in the bottom of the clay pot and sucks everything out, and same with the J tube.
I can't wait to do some cooking in here, and get the insulation/brick surround in place! Thanks everyone for the support!