From the Kay Erdwinn’s Post Teaching Yoga for Round Bodies
Because of extra flesh in the upper arms and thighs, it’s hard for a fat person to wrap one limb around the other.
If the student has good balance and is otherwise fairly flexible, I have him/her place the foot of the raised leg on the lateral part of the knee, rather than the back of the calf. This allows more room for the thighs, although admittedly it sacrifices some of the twist. It does provide the same ankle strengthening and balance practice that the full posture does, though.
For the arms, I simply allow whatever approximation the student can perform while stressing that the forearms and hands should be (close to) vertical even if the full wrap-around is not possible. This prevents the upper body from losing proper alignment.
You will likely need to use a strap to “unite” the arms; that will also help with balance. Remind those who use straps to keep a loose grip with the hands.