for those without welders here are a couple ways to find center without extra tools. Sorry for the light lines.
1. with just a square find the diameter then divide that number and mark both points on the edge of the circle turn the square over and align it with the marks and the measurments on the square.
2. use a compass , find the diameter divide in half then put thepointy end on the edge and draw an arc, move it anywhere basicly and draw another or a few.
3. The easiest way, buy a compass and blunt the point a bit so it doesnt poke a hole in the leather and draw on the back of you piece. In my case i simply installed the point backwards.