I use a NeoAir large in summer, the NeoAir with a Ridgerest under it in winter. This works for me, though I do toss and turn most of the night, though mostly on my side. I do ensure the NeoAir is fully inflated before actually going to bed (always seems to lose air between first blowing it up and going to bed a few hours later).
If you're trying to stay on your back, put a 'pillow' of some sort under your knees to elevate them some. By elevating your hips you're only contributing to your problem.
And lastly, I'll echo te-wa's comment. For three seasons I use my hammock more than anything else. As a chronic lower back pain sufferer, I've never slept better. Any time I ground dwell I toss and turn and wake up with some level of discomfort. Almost every time I sleep in my hammock I climb in, lay on my back, go to sleep, and don't wake up til morning. With no pain. You might at least give it a shot if ground dwelling continues to cause you pain.