Here is an example of one for Second Today. You should be able to simply change the URL to any local one to make HTML panel for almost anything. I pulled this off of an old file and I am not 100%, but I am pretty sure this is all you need in the HTML. I had made some panels for programs, but it simply leaves a blank page behind when you close the app because PanelManager.exe still keeps focus instead of going back to Today screen.
<html>
<head>
<meta http-equiv="refresh" content="0; URL=\Program Files\SecondToday\SecondToday.exe">
</head>
<body>
</body>
</html>