Here I'm transferring the hole location from the plastic to the angle iron. On the bottom side I marked some lines on the blue tape to aid me in lining things up. Then with the angle iron clamped in place, I marked my hole location with a pencil because a Sharpie wouldn't work for this.
 

Here I'm removing the height from one side of the angle iron (arrows). The first reason was for weight, and second is because this side is mostly unused, so why not. After machining I deburred all the edges and was ready to make another piece that will be welded to this one.
 

I had these two pieces laying around and thought they would work for this project. these pieces will end up with a 3/8" hole (about where the layout mark is) and another 1/4" hole that will be used for some pins. The blue line is where I'll be cutting them off at.
 

I'm using some shoulder bolts that will be used as the pivot points for this headrest. This is the area that will fold down when the headrest is not in use. If you're not familiar with this type of bolt, these are used in the mold making and die making industry (machinist world). Normally the ground diameter (shinny area) would go through a very stiff spring and would be used to retract some kind of plate so the part doesn't stick to the punch (this would be in a die making scenario). Anyways, I won't be using any springs but this will be perfect for my application. I'll talk a little more about shoulder bolts soon.
 

After finding the exact spot I needed for the shoulder bolt, I drilled my 3/8" hole. Now normally I'd put something like this in my vise to hold it, but with the angle iron not 90 degrees any longer, I just clamped it to my mill table with some wood under it and drilled my work pieces. This was a little more work doing it like this but it's really no big deal.
 

Notice the end of the shoulder bolt is just above the surface (arrow). With this design, you can tighten the bolt and nut down but it will still allow those pieces to move. With that being said, this will give the headrest the ability to move 90 degrees, or fold down out of the way.
 

Now it's time to drill the 1/4" hole for the locking pins. And to be able to drill though both pieces and still have both holes line up correctly, I'm using some paper shims that will go between the movable pieces. The paper will act as spacers so when the nut and bolt are now tightened, the two pieces won't move while I'm machining.