https://www.thingiverse.com/thing:1701228/ I printed one of these and it works great for plugging the power jack on the Arduino Mega. I had some tape over the hole, but this seems a little more professional.
That’s a useful thing. It’s not uncommon for the 12V cable to get into the wrong socket - I just did it myself this afternoon. @bar, that might make a good thing to add to the kit .