You are not forced to declare each element in a separate file, even if you chose to use XML for the definition of your site structure.
The XML site structure supports all the element declaration statements in-line. So if it doesn't make sense for you to have re-usable declarations, you can just centralize everything in one file.
For example, this is such a Hello World site structure:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE site SYSTEM "/dtd/site.dtd">
<site>
<element id="HELLOWORLD" implementation="tutorial.helloworld.HelloWorld" url="/home"/>
</site>