In practice (sample=1), I remember my Trek Madone from like 2006 or so came with these little rubber bar plugs that suspended a small (maybe 5/16 dia x 3/4" length?) rod in rubber webbing. I was using 44cm alum bars and 23mm tubulars on that bike and I couldn't tell a difference with them in or out. They were Bontrager branded and looked pretty cool so, I guess that's something.