This plugin has worked very well except for on my Kobi. The cover images are very small. This thread has some pics of what it looks like.
https://www.mobileread.com/forums/sh...d.php?t=280603
One of the posts there mention the problem is the lack of a percentage setting in the cover html.
In epubmerge.py I changed <img src="cover.'''+coverext+'''" alt="cover"/> To <img src="cover.'''+coverext+'''" style="height:100%;max-width:100%;" alt="cover"/> and it has been working much better for me.