Here's an alternative which may be...slightly unorthodox, but see if it works. Use a strap. I use one. I used to play a lot of electric guitar so I used a strap, but when I started playing acoustic more, I started walking around playing my guitar at school and in the streets and such so I still used a strap in order to play while walking. Using a strap, I would practice standing up most of the time, but I wanted to practice while sitting down, since it is a lot more comfortable. However, I found that the guitar would be in a different position while standing up and while sitting down. Holding the guitar in two different positions requires practicing everything twice (once for each position), and I obviously didn't want to do that (we all know the grueling practice diligence that is required to become great even in one position!). But I found that if I wore the strap very short, then I could achieve the same position while standing and sitting. This became very handy, as I can transfer between sitting, standing, and even walking without breaking rhythm. It also helped my posture, as maintaining this position requires sitting up straight, but is actually very comfortable once you get used to good posture.

Transferring into the world of flamenco recently, I naturally decided to simply maintain this same position for flamenco. I actually stumbled upon your post while I was browsing out of curiosity about what is "supposed" to be proper sitting position. I saw some post somewhere that recommended one should watch Paco Pena for reference to a more traditional sitting position and Paco de Lucia for a more modern position. Paco de Lucia's had actually been one position I was considering before deciding to simply continue my strapped position, except it would not allow that same freedom of movement from sitting to standing, etc. But Paco Pena's, I found, was actually very similar to my own. If one looks at the relationship of the guitar to the body and legs and the angle of the guitar and such, it looks very similar to mine, so in a way my position could be considered to be correct from a flamenco's point of view. The advantage of the strap, however, is that no support is necessary from any part of your body, be it thigh, hip, armpit, whatever. The guitar just hangs. It is balanced, so even if you stick both arms straight up in the air, it stays right in its place. And since the strap is so tight in order to make it short, it doesn't really move much to any side. This all gives you complete freedom of movement in both arms, as well as both legs if you want to walk around and play flamenco and be the new flamenco revolutionary

. I have just started playing flamenco so I have yet to see if this seems to be a good position for having complete power and control and such, but it seems to be working pretty well for me so far. Perhaps it will be advantageous to you as well. Buena suerte, amigo.