/etc/skel might be a good place to start, but I've found out that the actual default settings like panels, items on panels, etc. are stored in subfolders and files under "/etc/xdg/"
If you want to have different items on your panels, it's probably the best to start an XFCE environment somewhere (e.g. on a live distro (most comfortable using a virtual machine, like qemu)) and modify it to your needs.
Then copy the ~/.config/ folder to a safe place.
Inside that ~/.config/ folder, you will find a structure matching that of /etc/xdg in the UCK's remaster-root. You can now copy/paste config parts as you like it.
For adding/modifying items on a panel you must copy the corresponding xfce4/panel/*.rc files, and make sure that you're referencing to the right ID within xfce4/panel/panels.xml. Here's an example:
Entries created during an XFCE session often get unique IDs, so the entries might look like this:xfce4/panel/battmon-6.rc (Battery monitor)
must be referenced within panels.xml as:
<item name="battmon" id="6"/>
No matter how you prefer that id to look, just make sure the id in the XML matches the one in the filename.<item name="battmon" id="12430093470"/>