very close to the opening for operators to load, and for the robot to pick up.
This is the major concern. I've used two light curtains in the same manner; last one has been running 10 years, standards probably have changed.
Operator in zone, remove robot ok to enter permissive. Operator can never pass through light curtain to be within work zone.
Light indicating ok to enter zone to operator.
Light curtains wired to safety relay so if both light curtains tripped, e-stop, requiring safety relay reset. Light curtains auto reset.
The stop time/distance is the biggest thing to be concerned with.
Thoroughly test and document stop times.
In my opinion, the most dangerous activity with a robot is being within cell using the teach pendant. Too many examples of people being crushed.