How can I create a custom child to a panel

Hi, I'd like to create a modular panel using XAML/C# and then embed that in a stacking panel / docking panel etc. In Java I'm used to the ability to create any number of Jpanels and then keep nesting them .. but so far in C# WPF I haven't found a way to embed something custom.

 

Another way to ask this .. it seems most of the basic tutorials / examples have a single XAML file to reprent the full UI, and everything is embedded into it. I'd like to modularize my program by having sub-panels that each have and XAML/C# representation.

 

In solution explorer I've tried a bunch of things in add->new item, so far there is nothing I found that has an XAML reprenstation that can have a parent in my main window.

 

Thanks in advance for any help!