How to open up a Hydra controller?

There's something rattling around in one of them and I'd like to take a look / fix it so that I can confidently use it again.

I see the three rubber stoppers on the bottom, and I managed to get one out using a knife and then I realized I definitely shouldn't do that with a knife.

I then tried various tools and can't either of the other two out. Is there a trick to this?