How should you write a formal thank you letter to someone that barely knew you but still opened up their home to you and asked for nothing in return?

First, a hint: for images such as a template, you really need a search engine, not a site like this. Here's a link to one of the sites that popped up when I typed in "template for formal letter"...

Second, you write a letter the same way you talk - put down all the things you'd like to tell these people and close with something like "Fondly," or "Thankfully."