From the Kay Erdwinn’s Post Teaching Yoga for Round Bodies
As with pregnant women, fat people need to accommodate the belly via less bend in the leg that crosses over. The difficult bend is at the hip joint, not so much in the knee joint.
Holding the knee of the crossover leg may not be possible. I personally practice this twist with straight legs (not locked) and holding a strap around the foot that’s crossing over. One must make sure the student isn’t tugging at the strap and putting extra strain on the knee, though.