Ideally you want a small piece plastic glued to the bottom side of the thumb pad that it just touches the switch. You can have a gap between them but you will just have to push a bit harder.
So, yes, you're just flexing the plastic thumb pad. I've built several of these and it's a great kit.
So, yes, you're just flexing the plastic thumb pad. I've built several of these and it's a great kit.