Hi @Xfixiate as @MitchellMc highlighted, fixing a whaler directly to the corrugated sheet will not be structurally correct as you will get compression and flex the sheeting. Building a pergola itself will most likely require a building permit, which would require drawings outlining exactly what will be required. Having said that to give you an idea, Cutting out the Colourbond and fixing the whaler to posts that are structurally fixed into the ground with regulated spacings based on length. A local draftsperson would be able to provide this service, arrange for the permit and allow you to then build the pergola with all the provided dimensions and materials.
Regards, Nailbag
