Custom Timeline Playable - GUI Override

Hi All, I have written a simple custom playable with it's own track etc for timeline. I'd like to be able to change the GUI for it in the timeline editor to represent the data. Example shown here...

The clip on the left is default however I'd like to create my own GUI based on the Rect of the clip. Any ideas where this would be overridden? I was thinking likely OnGraphStart or something but I'm not able to work it out.

Thankyou!