Installing a stringer on a foam board that's already been shaped out of the blank

Hi

First post here. I am trying to reshape a beaten up wavestorm into a midsize board and I want to install a stringer on it before I glass it. The only problem is that I have no idea how i would clamp the two halves of foam together in order to set the stringer because the board is already shaped, and doing so would leave huge dents in the rails. 

Any recommendations? Should I just skip the stringer and risk snapping it in half?