1. Put it on a level surface.
2. Put a bubble level on it.
3. Shim it until it is level. Mark where your shims were placed.
4. Measure the shims, get some metal the same measurement as your shim and weld it where you marked it.
Too easy and fun.
I had a similar problem. My keggle wouldn't sit steady on the burner. I just took 2 strips of metal (2" x 6", 16 guage) and welded them underneath on the outer portion. Kinda' like rails. When I get home I can take a picture and upload it. Now my keggle sits firmly on the burner.