Although there is no "one size fits all" addendum for book contracts, here are a few resources:
The Model Publishing Contract for Digital Scholarship which includes a main contract and multiple schedules which you may be able to adapt to your needs. This is CC0, so feel free to use in any way you see fit.
The TOME (Toward and Open Monograph Ecosystem) website includes various resources, one of which is a template for an amendment to a traditional monograph that is to be published open access. The amendment is intended to be used when there is a book contract already in place and the press and the author agree to make the book open access with the payment of a subvention. See https://www.arl.org/resources/the-tome-summit-2018-amendment-template/ for the amendment.
source: Lisa A. Macklin, Director, Research, Engagement, and Scholarly Communications, Emory University Libraries, SCHOLCOMM listserv, June 25, 2019