They key is to put yourself into the mindset of a hurried deliveryman and design your system around that.
I have a sign low on my door right where a deliveryman would leave a package that asks him to put it in the box (with a simple message in big text, a big red arrow pointing towards the box and a photo of it with a package inside). There's no lock to get in his way.
Deliverymen from all carriers use my box about 90% of the time they want to deliver to my doorstep.
For Amazon specifically (since their deliverymen are gig workers), I entered address-specific delivery instructions into their system, mainly so I could complain about their performance more effectively. I think the actual, physical sign is more effective.
I have a camera watching the porch and have a few videos of deliverymen doing a double-take on the sign, then putting the package in the box.