Yes I was talking about using a premade sheet and strength wasn't really a great concern although I do agree that doing the whole thing as a complete unit would be stronger.

All a gantry has to do strength wise is hold it's own weight, the weight of the spindle + it's fittings and resist relatively small amounts of lateral forces and the occasional shock load, a single piece of 6mm plywood can easily cope with the strength requirements without failing.

The problem is rigidity and coping with those forces while still being in the correct position all along it's span to the nearest thousandth of an inch. Which is why I was thinking of CF sheets with the honeycomb acting like a torsion box, the strength of CF isn't needed but it's insanely good stiffness & vibration dampening properties are.