Benro has announced a lens adapter with 2x drop-in filter slots that allows filters to be changed without unscrewing the optics (such as the Aurora Aperture Converter. The Benro Aureole adapter will appear in two versions to connect Canon EF lenses to either a Canon R-mount or Sony E-mount camera.
In fact, something equivalent is already available from Canon itself (for EF on R), but for - the Benro Aureole will cost ( if pre-ordered) and offers a dual filter slot, one from the side and one from the top. So two filters can be used together. Available filter options are ND, CPL as well as grayscale, with the ND filter apparently designed to be variable as there is a wheel to adjust the strength.
Weight for the adapter is listed as 130g (presumably without the filter). Contacts for autofocus/aperture are present. Unfortunately, it is not clear from the announcement which filters are included, nor are the two adapters available yet.
The idea of attaching filters behind the lens has its charm, but you should keep in mind that with an interchangeable system like this, there is repeatedly the risk that dust or the like gets into the camera or onto the sensor.