I use the following - I have it set as a template so Sigil uses it when I click on Tools/Add Cover
Code:
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>Cover</title>
</head>
<body style="margin:0; padding:0; background-color:black">
<h1 style="display:none" title="Cover"/>
<div style="margin:0; padding:0">
<svg
xmlns="http://www.w3.org/2000/svg"
height="99vh" width="99vw"
preserveAspectRatio="xMidYMid meet" version="1.1"
viewBox="0 0 SGC_IMAGE_WIDTH SGC_IMAGE_HEIGHT"
xmlns:xlink="http://www.w3.org/1999/xlink">
<image width="SGC_IMAGE_WIDTH" height="SGC_IMAGE_HEIGHT" xlink:href="SGC_IMAGE_FILENAME"/>
</svg>
</div>
</body>
</html>
If kobo barfs on the vw/vh, first off - complain to kobo and tell them to get their act in gear
, second, just change them back to 100% (or 98% if you are having overflow issues)
Let us know if that works on your device(s) as well!