What about waiting till he's emptied the container - then cut the container apart to make a pattern? You'd have to add your seam allowances of course, but you'd have your shape. I agree with Sewwhat85 that your easiest method would be to buy a little pre-quilted fabric.
HOWEVER....if you used insulbrite (like for pot holders) and quilted it yourself, then it would keep the ice cream colder and his hands warmer.
Good luck.