All done, just requires a light pass with the grinder to clean up the swarf.
Here it has been carefully marked out for the bends and using a straight edge and the grinder grooved to make the bends easier.
Here the bends are complete and the mating corner welded.
The bottom cut out ready to weld.
The bottom welded. I tacked the corners and ran 1" welds every 2 1/2" to prevent warping and since a tight air seal is not wanted. I drilled the four corners 7/16" for ventilation and to drain any moisture that might find its way in.
So after drilling a 1" hole in the end, for the electrical access, I welded the box to the sawmill frame. I got both batteries and checked the fit, perfect.
Here is another view. Next job is to make the top. It's not supporting any weight so I will go with a lighter material.