fabrication question

Okay.. I need the help of a gear head here.

I am fabricating a switch panel. It's going to go where the factory locker switch is located now. I'm putting a lot of effort into making sure it looks as close to factory as possible when I'm done with it.

I'm taking styrene and heating/bending it to conform to the factory shape. My idea is that it will protrude slightly out of that opening (completely replacing the factory switch/power plug/big square useless faceplate area), but match the overall outer shape. I was looking at the curved corners and originally thought they looked about the same radius as a piece of quarter-round. So, I made a template and curved the piece with the quarter-round. I was wrong. The quarter-round was too wide of a radius.

I'm trying to figure out how to determine the size of that radius so I can make a jig that will let me bend the material to match it. I really sucked in Geometry. I think that's affecting me here

Being that the factory panel is not square I've had no luck drawing it out.

Can anyone suggest a way to either accurately determine that radius or suggest a stock piece of wood or metal that might be close? I thought about a pencil, but I believe that's too tight.

Thanks for the suggestions. Blasted compound curves and shapes from injection molding.. So hard to re-fabricate in the garage.