The first step is to consider the size of the circles you need to find the center of.  I have made this jig so that it will work with circles of around 6 inches in diameter.  You scan scale this project up to the size you need. 

Now that we have a plan, let's build the parts.  This design is simple, we need to just make a right angle, like a square, and then add a diagonal that is lined up with the vertex.  

I created a rectangle 6 inches by .5 inches.  

Copy the rectangle we made in the previous step so that you have three parts.  Rotate one of
rectangles 90 degrees so that it is perpendicular to the original.  Using the X and Y values. Set both so that they have the same X value. This value is relative to where you have the parts on the canvas.  They just need to be the same.  Doing this will have both parts overlapping in the bottom left corner.  Select both and use the combine tool to unite the parts.

Select the L shape you just created and copy it.  You need this for the second layer of the jig.

Next, we need to move in our third rectangle.  Using the rotation property, set the value to -45, this will rotate the rectangle so that it is sloping up from the bottom left of the L.  Move the rectangle so that the top edge of the rectangle intersects with the vertex.  Please take note of how I have shown the alignment in the picture. 

Now select the angled part and the L and use the combine tool to unite.

Also, copy the L to a new canvas. ( you can optionally cut both shapes out on a single sheet of material.  I needed to use a different sheet in my case, which is why I copied it to a new canvas.  

Using Wood glue to adhere the two parts together,  align the two L sections so that they are perfectly overlapping each other.  This alignment ensures you are finding the center when the jig is used.