I've don't use thigh straps, I use twine. When ya buy ya whole chicken thighs, ya just put the dried apricots in the middle, roll them up and tie them with twine. Then bake them at 175 degrees for . . . Oh, not those type of thigh straps.
As an experiment, I made thigh straps out of some old webbing used for tie-down straps. I was able to tie them to some strong recessed points on my Feelfree Nomad. When testing them in the surf, I adjusted em so I could hook them over my knees. Then I 'held on' by trying to squeeze my knees together. My knees remained about 25 cm or so apart. This meant when surfing in and going sideways or whatever, I could twist and bend from the hips with no problem of being flipped out. When the yak suddenly tried to go one way and me the other, I could do a paddle brace or bend without suddenly finding myself in the water. It didn't solve all the problems but my experiments showed me that the straps can be useful. Like even one time when playing and paddling into a big wave, it smashed across the yak into my chest but I just held on with my knees in the straps. That's my experience anyway.