XML Sitemap on multiple sites error

My problem is as follows:

I have a couple of sites, created with Drupal's multisite option, running on subdomains like:

site1.domain.com
site2.domain.com
site3.domain.com
etc.

On every subdomain I need a Sitemap, but since this module writes the sitemap with a fixed name in the files directory of the main domain, it only has the entries of the site that was updated the latest. Even though it says the sitemap can be found at: site1.domain.com/sitemap.xml, if site3 was updated most recently the content of the sitemap on site1 is the content of site3 (since there really is only 1 sitemap). It's confusing Google :-D

Now i guess this could easily be fixed by making the name of the sitemap customizable so I would have for instance site1-sitemap.xml, site2-sitemap.xml etc. but I'm no programmer, so I'm not sure.

Perhaps I'm doing something wrong, or there is a simple solution for this, so any help would be much appreciated!

Best regards,

Ron