What's the best way to coach a young leg spin bowler to disguise their wrongun better?

I’m not a leg spinner but as a former wicket keeper I had the good fortune to keep to a number of really good leggies.
I shared your question with one of them who's also a highly respected spin coach and his feedback is.

"When you bowl a leg spin you present the ball to the batter with the palm of your hand and when you bowl a wrongun you present the ball with back of your hand.
To disguise it better, try and stay as side on as you do when you're bowling the leg spinner.
Also practice the holding and delivering the ball across the seam so it’s harder for the batter to pick the wrongun. "

I'd say get your obvious wrongun working well and turning then start swiveling your hand around and see what happens. A fast arm action helps disguise deliveries. The final flick of the fingers the 'other' way really helps so work on that in the backyard to strengthen the fingers. Bowl to a mate who bats ok and video some deliveries. You can also practice on your own with a tennis ball