I’m struggling with Warrior I.  To get a more honest hip flexor stretch, I prefer using Crescent.
You can even use the pose to help with low back pain if you tend to overarch your lower back (hyperlordosis).

From Warrior I, lift the back heel, and bend the back knee so you can tuck the tailbone under. Pull the  belly in and up, that helps to support the lower back.
Then slowly start straightening the back knee, while keeping the position of the pelvis. You don’t need to straighten it completely, just as far as it will go without lifting the tailbone up.

You should feel a stretch in the front of the back leg’s thigh.

I’m working on a post that explains what role the hip flexors play in chronic back pain–please subscribe so you don’t miss it!