I made a modified Firebox today to suit the stove I made for the Tibetan ti 900 pot, I used a good secondary windshield this time and got a rolling boil in 8 mins for 500ml.
I think my stove is at or near the limit as far as heat output is concerned. The firebox is smaller dia than the pot, going bigger would simply put heat past the sides of the pot so I don't need to go wider on the firebox, I could go for a taller firebox but I'm not sure if there's any point, the pot can only absorb so much heat and any additional output would be wasted fuel. As it is 2-3 x 6 inch lengths of 1"x1" firewood split in two and broke into 2" lengths is all the fuel it needs to boil 500ml.
The windshield is critical IMO, it needs to be close to the pot but not starve the fire of air, I'm still drawn to a hanging windshield/heat exchanger rather than one which sits on the ground. I think a mesh potstand is better than a semi solid one with a large opening to feed in fuel. The open section tends to act like a chimney and the flames really shoot out and that is simply wasted heat.
I'm still trying to source welded wire mesh (Hardware Cloth) for potstands and grate, need some sheet aluminium too. Both are expensive as I need to buy way more than I need. I suppose I could sell the surplus on ebay.