If you modified the model opt-in notice to include language about the consumer's right to revoke the opt-in at any time and how it can be done, then if you provide consumers with a copy of such a notice that they completed then that will serve as written confirmation.
Comment 7 of the OSC to section 205.17(b) of Reg. E states the following:
7. Written confirmation. A financial institution may comply with the requirement in § 205.17(b)(1)(iv) by providing to the consumer a copy of the consumer’s completed opt-in form or by sending a letter or notice to the consumer acknowledging that the consumer has elected to opt into the institution’s service. The written confirmation notice must include a statement informing the consumer of his or her right to revoke the opt-in at any time. To the extent the institution complies with the written confirmation requirement by providing a copy of the completed opt-in form, the institution may include the statement about revocation on the initial opt-in notice.
Opinions expressed are my own and not necessarily those of my employer. They are not legal advice.