You should plan a series of chops to induce some taper into the trunk. See the picture below. The top of this oriental hornbeam will be allowed to grow until the wound is healed. Then it will be chopped again slightly above the first cut. I'll do that 3-4 times in the ground before I move it to a pot. If the plant isn't hardy where you live, you'll have to use a large training pot. You can develop the main branches while the tree is thickening, but leave the fine ramification for when the plant is in the pot.

Al