67 Barracuda Carpet Issue

Hopefully one of the masters on this site can help me with my carpet issue. The carpet was replaced by the previous owner and fits nicely in the front section. In the back seat floorboard area, the carpet wants to bunch up and fold over when I push it into the corner. It seems like it is not molded enough to fit the floorboard contour - see pic below. I just took the car to Dallas Auto Interiors and they said there was nothing they could do to help- even using a heat gun.

Anybody have an idea on how to get the carpet in reasonably flat?

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