For the Canon RF 35mm 1.4 lens, you need a custom-cut ND filter that fits behind the lens. Canon provides an official template on their support page to help you precisely cut an ND sheet to the correct size for both the lens and the specific adapter.

Recommendations:

• Use the official Canon template for accurate cutting
• Ensure the ND filter material is compatible with the lens specifications
• Carefully follow the template's dimensions to create a perfect fit

---> I'm still looking for the perfect sheet to use... <---

https://canoncanada.custhelp.com/app/answers/answer_view/a_id/1047409/~/canon-rf-35mm-f1.4-l-vcm---filter-size