What Scott showed is the way to go. You score the foamcore on the ID and you can create a radius. I have done plenty of times making prototypes. And believe it or not, you can make it “very clean”. If I have a chance to go through my archives, I will dig out some examples.
But beware that your front and back edge lengths will be different.
Back vertical + back horizontal > front vertical + radius + front horizontal
You will need to adjust accordingly.