With aluminum rod inside it, tubing will not kink unless you bend it wel beyond 90 degrees....and the average roll cage doesn't have bends that extreme. For a good fit, I use my Dremel and round files to 'fishmouth' cut the ends like real race car fabricators do. I also use epoxy for the glue joints, applied sparingly with a small disposable brush. Rubbing alcohol helps clean up any goofs while the epoxy is still wet, and working over wax paper keeps you from gluing your cage to your bench(epoxy will not stick to wax paper).
Heat shrink tubing, slipped over and shrunk prior to assembly makes good padding as well.