SOAPBOX

A crate for packing soap.

Any physical or media platform which gives prominence to the person on it and the views they espouse.

"The madman obtained a soapbox which he stood on at the corner of Broadway and Wall street, to shout out his prophesy of the end of the world."

A talk about one's pet topic (or the topic itself), especially when only tangentially relevant to an ongoing discussion.

"He's been on his soapbox all day about the new football coach."

To give a speech from (or as if from) a soapbox.