When setting up to sit for pranayama, nearly any upright position can be used. The material in the "Imagery for the Central Axis" section in my book provides a universal set of instructions that can be used for setting up any seated position in for pranayama.

Remember to choose your pranayama seat carefully each time you practice. Be sure to adopt a position that enables you to sit tall with the head, torso, and hips in a true vertical line so that you can illuminate shushumna nadi through your study of breathing.