The general principle for choosing filler rod size is to match the material thickness, (for sheetmetal, at least), so .045-inch rod is perfect. I welded my panels for many years with 1/16-inch rod, not knowing smaller sizes were available. What a difference the .045-inch rod makes! A 1/16-inch rod leaves a bead that's way too big!

ER 70 S-2 rod is fine. The next time you buy rod, if ER 70 S-6 is available, it's a little better for welding on metal that may have traces of paint, filler, rust, or corrosion on it. Now, I'm sure you clean your metal very well before welding, don't you? But even so, it's sometimes hard to get the last traces of impurities off of surfaces, and the S-6 rod has more cleaners and deoxidizers in it.

Bob Drake sells plugs kits, You might also check with some local precision sheetmetal shops. I used to have them made by the hundreds, and it's not terribly expensive.