Here is my invention for putting under the knees in savasana:
I've never really liked the way my legs roll out when I'm lying on my back in savasana. It doesn't feel great on my knees when my feet fall open to the sides. I saw a variation on this prop setup with the ends of the blanket rolled under. That works pretty well, but in a moment of experimentation, I placed the blocks underneath instead, which seems to provide a firmer hold on the legs. The feeling is similar to strapping the legs together, but a little softer. You can still separate the legs slightly, and even have them a little rolled out, but with a boundary. And we all need boundaries, don't we?