On the astrophotography branch of DPreview forums I found a site called Aurora Aperture which sells clipin filters for use if you have an EOS R, Nikon Z or Sony A7 camera and use Canon EF or Nikon legacy mount lenses and the existing respective adapters (Nikon FTZ or Canon EF-RF).
http://aurora-aperture.com/2019/07/1...rless-cameras/