I don't think anyone has mentioned the Corvette, 1997 to present. In between the steel center tunnel and the steel hydroformed side rails the bolted-in floor panels are a sandwich of very thin fiberglass over end-grain balsa. On the Corvette Z06 the frame rails are aluminum and the floor panels are carbon fiber skin over balsa. I guess fire doesn't scare GM from using wood. By the way, before 1997, Corvette floors were steel, not fiberglass, in case you wondered.
Z06:
I still like the idea of welded-in thin steel sheet joining all the tubes together though. If you cut individual pieces of sheet to fit in the openings, rather than overlapping the tubes, the weight can be kept down, and you end up with a stronger, more homogeneous frame than riveting on some aluminum or somehow attaching wood.
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