Hip Flexor stretch that doesn't impact the knee?

[OakLeaf]

Then if you can get down onto your belly, you can fold and roll up a yoga blanket or beach towel into a cylindrical bolster just wide enough to fit inside your hipbones, and do self-myofascial release of the iliacus and somewhat the psoas.

A table, picnic table without attached benches, bay window or kitchen counter is taller and more stable than a bed, to do the leg-hanging stretch. I've never been able to do that stretch off a bed, because the mattress compresses asymmetrically and I just roll off the side. A platform futon, with sides on the frame, might give enough support, but those aren't usually tall enough to get much hip extension.