Maybe making an announcement "kindly keep reading materials or tablets on you person, not on your bag" during the other pre-boarding announcements.
Let it not be missed that on our way to boarding the aircraft we pass thru one, two or more security checks where we are required to remove everything from our pockets, our belt, our shoes whatever.
Pre-security check(s) I, for one, put everything from my pockets in to the side pocket of my hand baggage so once I'm in the clear of those jobsworth types, which is sometimes once on board the aircraft, I need to transfer everything back to my pockets again.