depending on what type of welding you will do on the fender patch, some want a very tight fit for tig welding and some will allow a small gap for mig welding, some use the sheet metal bodywork clamps but the little flat steel tang part that fits between the two pieces will leave a gap between the parts as thick as that steel tang part is, unless you gind out a short spot to fit the tang of the clamp and allow the body parts to fit up tight. another way, if you have cleco fasteners, is to mark where you would put in a cleco and then make a small 1/2 round divet on each part so that when the parts are put together the 2 half rounds make a round hole and a cleco can be used with a small flat washer behind for the cleco to grab into and to draw the parts together. the washer can be taped on the back side, glued on or spot welded on if the spot is in an area with no access to the rear side
https://www.amazon.com/Welding-Clamp.../dp/B0787XV8SX
just a thought