Covering over a hole in an electrical box

I am hardwiring a transfer switch, instead of using their plug. I have cut a hole in the side and put in a cable clamp. I would like to remove the plug to create more room. That will leave a 2 1/4" hole. Can I put a piece of aluminum flashing over the hole, fastening with screws in the same holes that the plug was fastened with? I can't imagine why this should be a problem, but the code can be a bit odd at times. A plug would be easier, but I don't find any 2 1/4" ones.
BTW, I posted last week about how to join my solid 10 cable to the switches' standed 12. My soldering iron wasn't big enough, and I couldn't find the recommended wire nuts. Well, I found I could solder some stranded 10 onto the end of the cable using a propane torch. The stranded 10 twists up nicely with the standed 12, and the extra wire makes everything easier. A torch is okay to use on wire, isn't it?
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cut
fastening
odd
switches'
You will need to go to an electrical supplier locally for the 'KO seal" Your dimensions are about right for a 2 inch ko seal. This will not be acceptable if it is rain tite and the hole is above the bussing. If it is on the bottom or below the buss it does not have to be rain tite. (at least where I do my work)
Could not find red wire nuts. Hummmm
I do not use a torch any more. I use a soldering iron when I absolutely have to do "tinning". The jurisdictions I work around get pretty pissy about torches. As long as you did not harm the insulation your good to go.
http://www.electricsupplyonline.com/prod/mulberry-knock_out_seals.php
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You are correct, it is a 2". Why does a 2" measure 2 1/4"? Probably ID of something that fits in it. Anyhow, the two electrical supply houses I tried didn't have 2" kos. For some reason I already had some 2"-1 1/4" reducers, so I drilled some holes in a reducer, attached it to the box with screws, and stuck in a 1 1/4" ko. Looks professional; only we know how mickey mouse it is.

I was taught that wires should stay together on their own; wire nuts were mainly for insulation. Obviously stranded wrapped around solid won't stay together on its own; so I was looking for the wirenuts with set screws. Yeh, I know; I am doing everything the hard way, needlessly. A red wire nut would probably be fine; worst that could happen would be an open circuit I would have to renut.

Darkens the insulation a tad, but it is all inside the box. Thanks.
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