as a humble MEP PPL and often SLC can I suggest the statement:
"for your comfort and safety please keep your seatbelts fastened during the flight even when the seatbelt sign is off"
is simply insufficient.
Most of your SLC have never heard of CAT and even if they have, most of them will not expect to end up against the ceiling, at worst they'll expect their cup of coffee in their laps like during normal moderate turbulence.
Some improved statement should explain the risks.
Also it is time that trolleys ran on tracks in the cabin, perhaps on the side of the aisle so that people can pass, because they are the hardest and sharp edged heavy items around and lethal in heavy turbulence.
Even that however will not prevent the painful Stewardess burns of the coffeee can which I once saw in a 737 in sudden turbulence. It can t be that hard to design quick latch/unlatch turbulence safe trolleys on tracks