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

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

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