That armrest looks really good but as soon as I saw the hinge, I realized that it would not hold the weight.  You can hinge a bracing arm under the front of the arm that will swing down and provide support when the armrest is down.  It will swing out of the way when the armrest is raised.  If necessary, the bracing arm can be hinged in the middle as well in order to make it long enough and still get out of the way when raised.

Bill B

That would work fine, but I have learned not to put more than the weight of my arm and a bit more on the arm rest.  It's primary benefit is that with the left arm supported and my  right arm hanging free, after an hour or so my back starts to hurt due to an old injury.  Leveling  out the shoulders and evening the gravity load does the trick.    F