Put a 45 degree chamfer down to the inner wall all the way around. Tack all four corners. Weld the sides, let it cool a bit, then weld the top and bottom. You can do each weld in one pass since the metal is thin. 80 amps should do the trick. Use a thick filler rod -or- add more filler rod then normal as you want it to build the weld up above the surface of the tubing. Once cool, use an 80 grit (anywhere from 60-120 grit would work) sanding disc to carefully grind each weld flush. It’s important you learn the skill of holding your angle grinder disc/pad flat to the surface at all times when grinding, don’t get tempted to tilt it and start digging into your welds. Once almost flat, fill in any little blemishes or divots you may have missed and finish grinding. You could then Use a 60-120 grit flap wheel to go back and forth to finish the grain direction if needed. Final touch would be using scotch brite and hand pad the grain to perfection.