What are the dimensions of the carbon fiber you are replacing? or... are they fiberglass? I think I created a spreadsheet one time for someone looking to match the stiffness between different materials for backpack struts.
6061 tube would be good. I would think bar stock would either be too bendy or too heavy. You could get some carbon fiber tubes:http://www.mcmaster.com/#standard-carbon-fiber-hollow-tubing/=yw5518
If you want to stay a bit cheaper, many people like to use arrow shafts (used or new/clearance if you can find them).