I just happened to have the seats out of my LIV and so shot some pictures of my armrest which is attached.
Design requirements:
1) Had to fold up with nothing in front of front surface of front seat back rests so the co-pilot can hop in and scoot across without hitting the armrest stand.
2) Similarly, no projection behind the back of the seat back rests to clear knees of rear passengers.
3) Light
4) Places to put pens and pencils, front and back.
It is made of 1/4 inch fiberglass/core stuck together in the conventional way with flanges on the bottom to tie it down. The armrest itself is on aluminum piano hinge. If you push down hard, you can break it off. (Been there, done that in severe turbulence, reinforced with more carbon uni top and bottom.)
It attaches with the four bolts that hold the hand pump, plus two forward bolts bonded into phenolic "wedges" which were then bonded to the top of the spar box with flox to get a precision fit. The four bolts are not far enough apart to provide attachment rigidity. (Been there, done that.) The forward bolts make it solid.
Note in the photo "Armrest 3" that the hinge bolt for the hand pump is long enough that some reliefs must be ground into the armrest stand to provide clearance. It is all a bit of a tight fit.
Any questions, drop me a line and I can fill you in on some subtleties discovered during construction.
Fred Moreno