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Why three shops? Bunnings' Metal Mate range doesn't carry 50 x 50 x 3 mm tube, 100 mm flat bar or 20 mm round bar. An aluminium supplier sells these and will cut them to length for you. Buying all the tube as one 6.5 m length is the biggest saving. A fastener shop sells stainless bolts by the box for about half the 2-pack price. Bunnings covers the rest.