These articles compare how opinions are expressed in English and Japanese.
Page 24 of this booklet (page 26 of the PDF) talks about some of the cultural features involved in expressing opinions in Japanese.
These resources could be used to teach students explicitly about Japanese culture, specifically the ways that Japanese people tend to express their opinions.
Note: Originally, I planned to use videos from Erin’s Challenge or Japanese Pod 101 to explain about expressing an opinion, but I wasn’t able to find any suitable ones on the Erin’s Challenge page, and the majority of the Japanese Pod 101 content was behind a pay wall, so I couldn’t get at it without paying for a subscription. Because of this, I had to search for different materials to use.