ftp: How can I upload a site map on my Google Sites site with a custom URL?

I created a site for an electronic portfolio using the new Google Sites. Google sites automatically generate a sitemap at sites.google.com/yoursite/system/feeds/sitemap, which can then be sent through the Google "Search Console". However, I migrated to a custom URL, so I no longer have a site map in that location. Google Sites does not support FTP, and applications such as FileZilla can not access it. I have heard that SML Cloud can compensate for this, but I would prefer not to pay $ 5 per month just for a site map.

One of the ideas I had was to put my sitemap in a domain different from my site, but I'm not sure if this would work, or where I could do it. Also, if I need to finish making my website and then export it to a different host in some way in order to use FTP, then I do not mind doing it either.

Is there any way to place a site map on my Google Sites site that has a custom domain?

Thank you,
Calan Smith