Personally, I think you should use some carbon fiber tubing. It would provide a stiff but compliant support, and soak up any road buzz. Plus you would save a lot of weight and probably increase your speed by 1 mph. A second choice would be a titanium tube, but it might be difficult to drill holes in. Nix the aluminum -- too much vibration, it dents easily and can corrode if exposed to sweat or salt air near the coast. Steel is out due to rust issues, unless you can find some stainless steel tubing.