I'll answer my own question since I did some research and got really good results.
I used a deep, 14" oven and cooked 2, 4 to 5 lb prime rib roasts in it.
First I covered the bottom with about 1 to 2 inches of rock salt and sprinkled a little water on it.
Next I laid the roasts on their side in the salt. I had already seasoned the roasts with salt, pepper, and garlic.
I then completely covered the meat with rock salt and sprinkled more water on it.
I cooked it for 2 hours and 15 minutes with 30 briquettes on the top and 25 on the bottom. When the briquettes got down to about 1/2 size I added 15 more on top and 13 more on the bottom.
When it was done, I had to break into the salt with a hammer. The Prime rib roasts were done to perfection. They were very juicy and were slightly red in the center.
For one roast, I would use a deep 12 inch oven and cook about the same. The goal is to cook at 500 degrees, 15 minutes per pound.
Sixgun