I agree with F3G about finding another seat. If none available - then simply stand in the aisle one or two rows behind yours. This makes the cabin insecure and will ensure that the CC rapidly join you!!
If the situation is very bad you may be allocated a seat elsewhere on the a/c for landing. There may well be open seats that you cannot see with a quick heads up. But my guess is that, reporting on the spot is the only route. The advantage of writing is that you may get a clear statement of policy from the carrier, which can use if there is a next time.
Given human behaviour, there will be a next time.
