One of the great things about Nokia phones from way back when was making it your own with swappable plastic. While that level of exterior personalization isn't available on many handsets these days -- instead we've got bumpers and cases -- Nokia added a removable shell to its Lumia 820 Windows Phone. You can always go ahead and buy new ones, but if you've got access to a 3D printer, Nokia has just released a "3D-printing Development Kit" with all the documentation you'll need to create a custom backing for yourself. It's available for download from the source links below, and contains "3D templates, case specs, recommended materials and best practices."
Microsoft Windows Phone 8