Here is an example you can look at:

https://github.com/jspark311/SX8634-Breakout

I worked up the touchable surfaces as a footprint with 12 pins. That way I wouldn't have to worry about changing it if I shuffle other things around. The pads that are roughly square are easy. The chevrons on the slider, I needed to draw using the polygonal pad tool. But it wasn't that time-consuming.

The one thing I'd change going forward is the ground plane fill. For lower power usage, it ought to be cross-hatched. But my version of KiCAD doesn't support it.

You could make your own footprint!

I haven't used KiCad, but in Altium-based tools I'm familiar with, I'd use the fill tool in the footprint editor in whatever shape  I like (or import an image from some external software), make a schematic symbol as well, and then you have the "capacitive touch component" you can re-use as many times as you like.

There are guidelines about what kind of sizes or shapes work best, but they are only guidelines - most shapes will work, to some degree

Your comment made me take another look at it. Thank you so very much.

I've just embarked on a touch-input sound maker, so I'm curious about this as well!

As Jarrett wrote, you can use a fill tool. In the footprint editor on KiCAD 5 they have a graphic polygon tool. This can make an arbitrary shape, but defaults it to the  f. silkscreen. You can shift that onto the copper layer by having your polygon selected and press e (or whatever key you have bound to edit). A window should pop up saying that DRC won't cover it, just click through and you will have your shape in copper, ready to be used until the cows come home.

Thanks! I've give that a try. I'm still in the breadboard stage but PCB layout is hopefully coming soon.