Have you ever had Polaroid film that was meant for a different camera? Impossible Project makes a neutral density filter that will allow you to shoot their 600 film in the SX-70 model. The Lucky 8 edition film only comes for the Polaroid 600 camera models and I really prefer shooting with my other camera instead, so the filter was a no brainer to pick up.
The ND filter brings the 600 speed to approximately 100 ASA, it’s reusable and easy to apply. The great thing about this filter is that it does not require any hardware changes to the camera.