Aurora Aperture has a started a Kickstarter campaign around a very innovative product — they have created a set of drop-in filters that are compatible with the basic Canon EF-RF mount adapter, in addition to Sony E and Nikon NTZ mounts.
Their Adapter Mount Format (AMF) filter comes in several variants, including UV, various strengths of ND filters, graduated ND (GND) in 3 different strengths, and a light pollution reduction (LPR) filter. All are available for Canon EF-RF adapter.
Support the project by pledging and you can save 30% or more based on how many filters you order. One filter is $44 (much less than the cost of the Canon ND adapter). Ten filters are $34.80 each, a great savings from the expected retail price of $52.
Check out their campaign on Kickstarter today!