There is relatively ZERO tension against the "crimp." The force of tension is at a right angle to the crimp. Therefore, it doesn't take much of a crimp to keep it in place.

That said, I take much care in making the crimp. To start with, I see that the rounded edge of the hole in the plate is toward the crimp to offer more surface area and to prevent a sharp edge that can cause a crack. You should pay attention to this regardless of the tool used.

Secondly, you start peening in the middle to force the edges out evenly, then work outwards in a circle.

If, of course, one were not experienced with a hammer, this would be beyond their skill level.

Good luck,