This is how i sealed my Steam Buns. Hold the dough in the left hand, pressed the filling with your left thumb, while gently turning and folding the edges with your right hand.
Note that i did not remove my left thumb from the filling until i reached the end of the dough.
This will give you a nice presentation when steamed.
It takes patience and practice. I did lots of practice in sealing the buns with the playdough and i think so far I am getting there, a little more practice.
Practice makes perfect.
Have a nice day!