I've been mining gold info out of this thread so thank you to all who contributed. Here's what Charles said that I cut, reorganized, and pasted:
Extra thanks to Charles R. Stevens for forge details and experimenting.
Dig a straight sided trench, 4" wide and 8-12" long.
The tuyere comes in the side of the trench.
The long sides of the trench slope down to the tuyere making it easy to clean and encouraging fuel to settle down.
You would have 2" of dirt between the fire and the bottom and an inch under the tuyere.
You want at least 3-4" above the top of the tuyere to the top of the table.
Now at this point either use bricks or make mounds of dirt on the long sides of the trench up to about 4" to keep the fuel over the stock from running off. Banking coal works, but with charcol you just end up with all the fuel on fire, and still only get 6" of hot spot .
So with charcoal and a 3/4-1” tuyere I find a 4x8” trench about 3” deep to the top of the tuyere good. That makes the trench from 4-5” deep. (5” gives space for coal slag to collect). With charcoal you need somthing to bank the fuel pile above the hearth (and over the fuel against) so you need a 4” bank on on or both sides of the trench.
With coal you can bank against more coal, and a 6-8” round bowl works well but as I like the multi fuel aspect I use the trench for both.It also holds just a little bit less fuel, the sides make holding the fuel in place easier and it reflects the heat.
The trench is easy to clean and seems to give a better fire over all.
Down side of the double walls is that scrolls don’t fit well. Single wall works well and one can always grab a brick.////////
I know it's not perfect but it's what I have for now......may add in the future. I look forward to finishing my forge and will show y'all when it's done!