I just came across 勉 (べん) and the mnemonic is Ben whereas for 便 (べん) it was Bento. Would be confusing to recollect.
Whatever suits the best your imagination and the Kanji’s meaning i guess?
You can still add your own. 
To make things even more confusing, in Japanese language, “bento” is written with ateji 弁当. However, in simplified Chinese, it’s written as 便当 (biàndāng, trad. 便當), and the 便 part really does mean “convenient”.
I can see how that can be confusing. Though, maybe to add a different perspective, I personally don’t really mind it. Especially when it comes to the people (kou, gen, ken, kan, etc.) the characteristics and the “things they do” add up pretty quickly and at one point it becomes difficult to differentiate between them. That’s why I am glad when I can give half of the characteristics to one thing/person, and one half to the other. That way one mnemonic doesn’t get too overloaded.