The simple way is pretty simple -- ND .6 (2-stop) filter over the lens, and learn the trick of loading the 600 pack into the SX-70 camera.
You do lose two stops of viewfinder brightness, but that's reportedly not a big issue in sunshine, and even less so with a sonar camera.
I've been thinking about a way to put the filter over the film, either permanently mounted inside the camera (below the moving mirror) or over the opening in the film pack and use a fresh filter for each pack (to control dust, scratches, fingerprints, and wrinkles). If it works, this would be about as simple as the filter over the lens, but wouldn't affect the viewfinder brightness.