To get started paste this code into your custom page.
<p>Your browser does not support iframes.</p>
</iframe>
The Iframe code assumes that your custom page and sitemap are in the same folder.
I placed mine right after this chunk of code and it looked good.
Yours maybe different because of the original source code. Don't be afraid to experiment with the location. You'll have to adjust the height and the width of the Iframe to fit your code from your map so it all appears on your custom page.
It's easy to overlook something you're not looking for.
This is a site I built for my work.(RSD)
http://esmansgreenhouse.com
This is a site I built for use in my job.(HTML Editor)
https://pestlogbook.com
This is my personal site used for testing and as an easy way to share photos.(RLM imported to RSD)
https://ericrohloff.com