I have always had the habit of placing the chuck key on top of the headstock. The lathes I learned turning on were quite large machines with a headstock tray included in the casting, but now my little C3 does not have one.
So just getting on with it. Some sketching and cutting some 1 mm steel sheet to make a shallow tray.

Figure 1: The tray
The corners were secured/smoothened with a couple of epoxy glue dabs. More user-friendly then, and after painting in shop color, I fit it with two strips of double-stick tape (preferably the carpet type, or even better, some sections of those double-stick foamy pads).

Figure 2: Tape in place
Then just stick it in place and as can be seen, it uses the whole area available on top of the headstock.

Figure 3: Ready for use
Time to fill with suitable parts like the chuck key

There is actually room for the additional set och jaws, some oddities and the chuck key.

Figure 4: In use
Not much to talk about really, but ever so useful as it is much easier to find the chuck key