There are no sample cover letters. If you choose to include a cover letter in your application, it would include information that is very specific for your relationship and your file.
You don't actually need a cover letter, but a lot of people choose to write one to help the VO understand how the file is organized (where each form is located, what kind and how much evidence is included). You can also include any other information you need to highlight for the VO. For example I don't know my UCI from when I moved to Canada as a child, so in our cover letter I explained why that information is not there and provided alternate information for them to use to pull up my file.