We showed Cleco clamps earlier, but didn't really explain, so shot some pix. They are mainly used in aircraft rivet work, but are handy for any sheet metal work.
They are a spring loaded clamp and special pliers. When depressed/extended they become narrower, allowing them to go through the hole, in this case 3/16". When released they expand, aligning the holes and clamping the metal pieces together. Sheet metal screws can do the same job, but these are handier, especially if the part needs to taken apart a few times for fitting. When done, simply weld up the holes.