Find a suitable wedge, probably wood, and tap it in the slot in the clamp. Cut the ends off the wedge. The clamp needs to be about 1/8 inch wider.
Now you should have a slightly oval hole for the bars to go through.
If you lok at the bend of the handlebar you can see the the bend is quite oval. Line the oval section of the bend with the now oval section of the clamp, turning the handlebar as it goes through.
Better if the stem is a bit warm.