Here it goes the fotos and some descriptions:
1 - The V shaped metal plate:
It's a 10x10cm in each square and I can't tell you the angle because I forgot to measure it but that is something easy to do also. The plate already had some drilings on it but I had to do and extra two for make it perfect for the existing holes in the car floor. You also can see four screws attached to the "vertical" side of the piece; these were soldered (superglue doesn't work here).
2 - The metal piece in place:

I had to change the original screws for two stronger and longer ones in order to make it firm and steady. Don't worry about the handbrake adjuster because it doesn't touch in anything.
3 - The armrest drill holes:
I toke advantage of the existing holes in the metal plate and use it to mark four holes in the back of the armrest.
4 - The armrest in place:

Here the only thing that's missing is the nuts in the end of the screws to tighen it all up.
5 - The bolts in place:
The nuts are polished steel and they go well with the chrome in the door handles.
6 - The final result:
Final remarks:
This was the fastest, easiest and cheapest way I found for resolving this problem (only 5€ of material). With the right tools almost everybody can do this within one hour or less. The armrest is rock solid and doesn't wobble.
I hope that this can help all of those who're having troubles with their armrest.
By the way, the plastic of the armest sucks even more than the stock one on our cars. It's really easy to break it or scratch it so be careful.
The next step for me is to clean all the car and apply some plastic tratment product on all the plastics.
Bests regards for everyone.