Why a drink can still tip on a cushion
A wider base cannot repair unstable support.
Recheck after sitting
Seat compression changes the local slope. Set the drink down only after the intended seating arrangement is in place.
Check seating in the fins
The container must seat fully. A low handle or added boot can keep the bottom high even when its measured diameter is within range.
Change the location
If the holder rocks or the seat moves around it, move to a level table. Do not add padding beneath it to create another unstable layer.