I recommend a few things:
1. Study Aaron Shearer's "The Five Ways of Moving the Guitar" from Learning the Classic Guitar, Part One. When I first read this material decades ago, it was a revelation to me. I thought I fully understood positioning prior to reading this material. In spite of some of his writing overall being a bit dogmatic, this material helped me better understand how a default position is unique to each individual with various body shapes. It helped me better understand how positioning is a compromise between the hands both as a general default position, and as a specific position dictated by the particular physical demands.
2. In spite of the great information Shearer provides, he says very little about overall body posture. The chain is only as good as the weakest link. While Shearer assumes good posture, he doesn't provide the details that so many of us need as we recover from the habits of modern society. I learned a great deal from studying Alexander Technique, and I learned it in a very short amount of time. No doubt, this can be learned from other disciplines such as yoga, martial arts, etc. Speaking from experience, good posture is important. A very simple test: if you can not maintain good posture AND feel most comfortable while maintaining it, then you need some assistance discovering better posture. Without being able to find good comfortable posture while simply drinking a cup of tea, no guitar position, device, or special chair will fully solve the problem.
3. Use the knowledge gained from above to continue working with the Ergoplay, straps, and other various devises that eliminate the need for the elevated left leg.
4. Most importantly in my opinion, elevate the back legs of your chair. This will take a tremendous amount of pressure off of your back. As an experiment, you can have a friend stand behind your chair while you are playing. Have them raise the back of the chair off of the floor maybe 2 to 5 centimeters. Maintain that position for a short while. Then, have your friend return the chair to the floor. You will then feel the tremendous pressure on your back that you have been dealing with every time you are in a practice chair. I know that at least some orchestras use chairs specially designed with legs that are longer in the back. I've heard the phrase "cello chair" thrown around. In my studio, I simply use discarded hardback books for the task. I'm sure a wooden board would also work.
Dr. Todd Tipton, classical guitarist
Cincinnati, OH, USA