Increasing lower back/hip mobility for properly performing front squats

I’ve performed front squats for 2 six-week programs now and I really liked them. Initially, I saw much more development in my inner quads, but, as my body got used to that new shock, the growth and progress in lifts halted. However, I discovered the reason was not periodization or other exercise choices (as I keep my training very fresh) but the position of my back. I can’t keep my back straight enough to perform the movement comfortably passed my plateau weight and rep range. I’ve stopped them for the moment because I can feel stress on my back and spine — which I’ve never felt in any other squat or deadlift. My mobility DID increase from when I first did them, probably about twofold, actually. But, my back still isn’t straight enough that the load is on my quads properly.

Does anyone know any stretches or complementary exercises I can do to increase my mobility in the lower back so I can keep a relatively straight back during front squats?

Thanks!