Correct oem look is about 1" long electrode weld along the bottom from each edge of the bracket.
On the top of the tube, weld from the outside edge of the bracket to about the same measurement.
It is the width of each strengthening channel on the outside edge of the bracket. No welding in the centre depression of the bracket.
If you have an arc welder with DC range, use that setting with a general purpose rod. The DC setting will give a flatter, more original looking weld.
There is a measurement to observe on the original drawing, but you may find it easier to fit you'r original fender and toolbox and put the screw through the hole in the fender. Email me if you need more info or pictures.
Hopefully you haven't got a Tiawan bracket. The holes are in the wrong position and they don't have the square nuts spot welded to the back.