You can actually just buy a cheap, generic strap like this one, remove the plastic lugs at the end, feed the strap itself through the slots on the SX-70 and make sure it's secured through the strap harness.

I've done this and it works well enough. Just keep in mind that you can't easily remove the strap when you don't want it attached, the way you can with clip-on lugs. These straps also tend to run a bit long, so the camera can hang lower than the standard straps. I've thought about trimming the extra length, but I worry about being able to fit a frayed end through the harness easily. Finally, if you want to do this with a different type of strap, just make sure the width is smaller than the slots on the camera.