I tack-welded each corner before I fully welded everything (both sides). This helps keep the warping down to a minimum. Once the tack welds were in place, I put the heat to them all.
 

Here I'm machining the welds flush with the frame. This doesn't have to be perfect because I'll be doing a lot of grinding and bodywork later.
 

Making Tooling

I'm getting ready to weld the sides together and to help with that I'm making some tooling. I squared this block of aluminum first and now I'm making that 45 degree angle the same length as the smaller corner pieces.
 

I'm using my surface plate to setup on because it's very flat. And with all the pieces getting welded that are already square, this makes sure I get the two pieces in the right place.
 

That fixture worked pretty good and now I have all the sides tack welded. This was the easy part, fully welding them is much harder because I'll have to deal with heat related issues.
 

Here I'm welding the sides together. Now this turned out to be a challenge, much more than I thought. To keep everything square, I'm using a nice flat steel plate to rest my pieces on. However, that flat piece isn't very big.
 

I ran out of real estate on that smaller steel plate so I had to use my surface plate. This worked great but I had some trouble getting the last piece nice and square. But to be honest, I never really did get every piece perfectly square, but I got pretty close.