Google Cardboard Apps in a viewer without button. How to click?

I'm considering buying a replacement viewer for Google Cardboard, but most of the plastic ones tend not to have a button, making it difficult to navigate through most Cardboard apps.

I'm aware of bluetooth buttons, but they seem to be a bit too expensive for what I want to use it for.

Is there a way around this issue (if any) that doesn't involve an extra purchase, and if not, what's the minimum price I would be looking at?

For extra info: The phone I use is a Moto X 2014.