Last season I just had enough brick on hand to stand a few rows of brick on their side, and had several rows all the way back to the stack to try to keep the flame against the bottom of the pans. The whole firebox was bricked up. The brick did help hold the heat in but it took a good hour to get the pans boiling to the stack. It also got extremely hot all the way back so couldnt touch it. This year I'm getting some insulating blanket to put in. I guess it should hold the heat in better, more efficient.

I did use a piece of curved metal from the tank to have a bit of sloped ramp from the grate to the lip of the flue area, its not much of an angle but does guide the flame up.

I was getting the best boil by actually building the fire up high enough on the grate to actually lay longer pieces of wood back under the front of the rear pan, and when it burned off the coals would fall back into the firebox. This is 2x8 and a long way for the heat to travel. my firebox is over 2 ft front to back and could be a little longer, but I think getting the fire blanket in will really help. I'm losing a lot of heat with just brick. Also need to get the forced air in. I didnt get the fan hooked up last season, no electricity but I'm going to figure out a solar battery setup this year.

Keep working at it. I'm sure you'll get something going.