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I built the rollcage and chassis for my '37 Ford ProStreet out of Plastruct tubing and rod. I used three different diameters to approximate the various sizes used in 1:1 cage construction. Plastruct's .080 dia. tubing replicates the 1 3/4 inch diameter that most cages are built from very closely.
Any decent hobbyshop should have a display rack with all of the sizes in stock.