Years of using a footstool can lead to serious problems with the back and hips. The best solution is a support. The classical guitar ergoplay support ($44.95 ) attaches to your classical guitar with suction cups which elevates the guitar to a comfortable playing position while allowing you to keep both feet, flat on the floor. I often play 2-3 hours continuously and the Ergoplay Classical Guitar Lap Rest has helped with discomfort in my hips.
Also available: Left Hand Ergoplay Model. Select from the "Orientation" dropdown HERE.