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My current favorite stretch is an isometric lunge. You get the benefit of stretching the hip flexors with static contraction which can also increase strength and strength endurance. Start at 15 sec and work up to a minute 3-4 times a day.
You'll also want to add some form of dynamic lunging to your pre-workout warm-up or the workout itself to develop some dynamic flexibility at speed. Tempo sprinting (60-70% effort) can do the same.
The foot stool would reinforce tightness so I wouldn't recommend it.