Try and find Paul Dang’s advice on chopsticks.
Basically, open your hand like you are holding a couple boxes of cereal to give it a sort of “square” shape. This helps keep the strings from sliding.
Also what can help is having the strings go over all the tips of your chopsticks hand (not just over the index finger, as in the standard chopsticks mount) …I think Gentry says this in the tutorial.
Lastly, it can help to also use your fingers to pinch the string. For example, for your chopsticks hand have the string not go completely over all your fingers. Instead, have the bottom string go between your ring finger and pinky finger (for example) and pinch the string a bit between those fingers. This will help really stabilize the strings on your chopsticks hand.