I've created a Japanese-English dictionary for Kobo:
dicthtml-ja-en.zip (38 MB)
Documentation and installation instructions:
http://perm.cessen.com/2021/kobo_dictionary/index.html
This dictionary is generated from scratch from free dictionary sources, and can therefore be freely distributed. It's also complete with pitch accent information, and has an index that matches conjugated verbs and adjectives when selecting words, which I had trouble with when using other dictionaries.
As far as I know, this is a major improvement over previously published Japanese-English dictionaries for Kobo e-readers. I certainly searched hard before putting the time into making this.
I hope it's useful to other people!
(I'm also planning to release the source code for the tool I wrote to generate the dictionary, which has additional options for people that want something a little more custom (e.g. also including Japanese-Japanese definitions). But I want to clean it up and document it a bit first.)