Yes, I really dislike the table/box thing. (Although they have "squared off" the border corners recently, I think, which does look better.) Editing the metadata is easy enough, as said above, by going into the Calibre Resources folder, removing the read only attributes of the two files and changing them to your satisfaction. (BTW because Calibre overwrites your modifications with each new install, it's best to put your modified files elsewhere - Preferences->Advanced->Miscellaneous. Open calibre configuration directory. In this configuration directory, create a sub-folder called resources and place the files you want to override in it. Maintain the same folder structure as in the Calibre resources folder.)
After fiddling, you can replace/insert the "jacket" and/or cover using either GUI Plugin Modify ePub or the native Calibre equivalent/s.
Having said all that, after my edits I get the really nice (very plain) rendition of the fields I want. This looks really nice in the Calibre ebook viewer: HOWEVER, because I am not conversant enough with xhtml and so on, my efforts don't display correctly on my tablet/s. The fields are populated correctly and show as expected, but I am doing something wrong that causes my CSS formatting to be ignored (in any of the various reader apps) meaning the error is mine. So, I am not going to hold forth about the actual modification of the CSS and XHTML because I am but an ignorant fool in that area. This has prompted me to go back and have a more in depth fiddle.** More below
Hope this is vaguely helpful.
** I uninstalled and reinstalled Calibre, cleaned every trace of it, including User data. There's good news and bad news.
The good news is I don't THINK I'm an idiot any more but am open to suggestion on that point. (My assumption having been that my modifications were to blame for the incorrect display on my devices.)
The bad is that when I run this clean install (ie with the unmodified Calibre template and stylesheet) then use Calibre Polish to replace the jacket in a couple of books the jacket header displayed in both my ereader app (Mantano on Android) and ADE on the PC is not formatted. The header table, background, borders and so on you see in the Calibre viewer do not show at all, and none of the font attributes -- other than the italics in the series field -- are displayed as per the stylesheet.
Can anyone confirm this?