Catch can on rocker 'hockey puck'

[Evil]

Looking to add a catch can to the pipe from the hockey puck (think that's what it's called) on the cam cover..
What size bung do I need for the other end or..
Would it be better to add one inline and keep the pipe to the inlet?
Is it ok just to add the can or is the system more complicated than that?
That thing in the pic, in case I'm being vague (sorry for the quality)..