To keep the warping to a minimum, I welded a 1 1/2" bead and then turned the box 90 degrees and repeated the process. Once I had four welds, I turned the box around and did the other end. This worked out for two reasons: first I knew that if I welded from end-to-end in one shot, I would have a warping problem. And the second thing, my welder is only rated at 20% - 25% duty cycle. This means I can't weld continuous because the torch gets to hot (the part in my hand). So after welding those small welds in eight places, I would let everything cool down for about 30 to 45 minutes and go back at it.
 

Fixing A Problem

Note the series of welds along the sides. I did this in six steps. To get to this point it took me two to three days. And as careful as I was, it still moved with the heat. What I'm doing is squaring the box again with the aid of a large lever.
 

As you can see I have the box clamped down and then using a large bar along with a piece of aluminum to pry against, I was able to move things around. Whatever works...right?
 

Custom Fitting

Once I had the box a little straighter, it was time to machine the back piece. I ended up with the box slightly out of square (what a surprise with all that heat) so after taking some measurements, I cut the back piece to fit the box, which means it will have a slight taper on one side. I needed .085" less at one end so I used a dial indicator to make that happen (arrow). Once I had the correct amount I clamped my work piece down and cut the angle I needed.
 

I did the same thing with the door frame but for some reason the front of the box wasn't as bad as the back was. I also cut the inside of the frame at each corner where some weld was.
 

Once I had the outer surface cut to size, I needed to cut the 45 degree angles at each corner. I used my 45 degree angle block (that is clamped down) to bank against, which worked out great. And notice the corner of the door frame touching the parallel next to the angle block. I used that parallel as my work stop.
 

I machined the back piece the same as I did the door frame, with 45 degree corners. However, what I didn't show you before was how I did the last corner of the door frame. When my work piece has a sharp corner, it's easy to bank against the work stop (parallel). But when you don't have a sharp corner any longer (after machining it away), I had to come up with something so the angle would be the same size as the rest.

With the use of my ridged scale, I placed the edge of the scale flush with the edge of the work piece (side and end) and then clamped the two together. Now I could use the end of the scale to simulate my corner of my work piece.