Balsa Wood Surfboard Rails and Inside Frame.
In the pic below the frame of the board and the bottom deck are fiberglassed into place.
all of the claps have a buffer between the clamp and the balsa wood because the balsa wood is so soft, it gets dent easy. i even use weights to hold some parts in place. I cover old sockets in wax paper so that they don't stick to the wood they are holding down. small fishing weights would work to.