public class DemoBlockEntity extends BlockEntity implements ImplementedContainer, WorldlyContainer{
    [...]
    @Override
    public int[] getSlotsForFace(Direction side) {
        // Just return an array of all slots
        return IntStream.of(getItems().size()).toArray();
    }

    @Override
    public boolean canPlaceItemThroughFace(int slot, ItemStack stack, Direction direction) {
        return direction != Direction.UP;
    }

    @Override
    public boolean canTakeItemThroughFace(int slot, ItemStack stack, Direction direction) {
        return true;
    }
}