I use an inflatable foot pillow similar to the one shown here:
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It takes the pressure off the back of the thighs and I've used it successfully on flights to/from UK and Australia. IT packs fairly neatly and doesn't take up too much space.
As well as not interfering with the seat in front, it has an added benefit in that it stops the person sitting next to you spreading their legs into your foot area.
Depending on your height, you may need to find the right balance between how much you inflate the pillow and where you position it, ie how close or far from you. I found too far out ended up putting pressure on my lower back, like I was slouching. A bit closer was the perfect spot for me and I got several hours sleep. This was on Cathay Pacific on the super quiet and comfortable A350. The big plus was the new head rest flaps that open out as well as fold in. Much better than any neck pillow and I could keep my bose headset on while sleeping. I'm normally a side sleeper but with the combo of foot pillow and great head rest I was able to sleep well on my back in the fully reclined (all 6" of it!) position.