suggestion box; opinion box
A physical box or digital form for collecting feedback, suggestions, or complaints.
A suggestion box is set up in the company lobby.
advisor; sounding board
A person who listens to ideas or complaints and gives advice; often used humorously or self-deprecatingly.
He always acts as a sounding board for me.
Compound of honorific prefix ご (go) + 意見 (iken, 'opinion') + 箱 (hako, 'box'). The figurative sense 'advisor' derives from the idea of a person serving as a receptacle for opinions.