As an employee of a software company they own my personal time projects, this is not in question. That said, my employer is quite reasonable about said things and at least verbally quite willing to review something and say "this isn't related to our work, you can have it."

It being a small shop, verbal acknowledgement has been all they used before; they have no form for assigning a particular projects IP back to an employee. For a project I want to make and open source I would like something in writing. My employer is receptive to that but they want me to provide the form.

I have been looking for either a template or some resource to help me write a little thing that they can sign that says something like: "The company transfers back all rights related to this project to employee, and we cannot take this back later."

I suspect I don't know the correct terms to search because all I find are templates for employers to have new hires sign that assign their IP to the employer or articles talking about how really the employer owns all IP anyways. 

Is there a particular term I can search for this? Do such templates exist?