Many years ago I built two panels for my Cessna 140. It wasn't that difficult with a couple of the correct size chassis punches, a small shear, and files and such. The most difficult part was rolling the bottom of the panel under 90 degrees for a small lip. I had the local junior college metal shop do the bend, they did the radius to sharp and I got to make another panel. I am by no stretch of the imagination a craftsman, but it came out very well.
Today there are a number of online resources that will help you design a panel and then they will cut the panel that you design.
Tom