Break it down into a multi panel layout with straight line segments that approximate the shape. Then you can use a MAC calculator that does multipanel wings to determine the MAC. From there start with a 20% of the MAC location for your CG.

Or build a little test glider and play with it by moving the CG around and playing with the reflex on the trailing edge until it glides in a stable manner that can recover from stalls but doesn't recover TOO quickly.