[Bugeyev8]

I would go back and fill the pinholes , just weld them up so water will not get behind your filler and paint

as far as the gap, just get a copper pipe fitting or piece of copper pipe and flatten it to shape fit to your roll pan, use a c clamp or something to hold it in place and just fill the gap in

those HF welders don't work great for sheet metal because they run the flux wire and thats usually 0.35, sheet metal 0.23 or 0.25 is better and they don't make flux core that size

either way...what you have done so far looks good, just don't get in a hurry