Actually, the “door” in question is actually not a door at all!

If we turn this “door” clockwise we reveal…

…that it is actually a door frame!!

It is a piece of paneling from the doorframe in the First Class lounge based off a piece that was actually recovered:

This piece isn’t more than 3cm at maximum thickness!

In reality, if the panel in the movie was based off the actual panel, 

It shouldn’t have held either of them afloat!!