I used 1/2" X 1/8" stainless because It's available at work. I welded a thin ss flat washer to my tabs and ran the clamp bolt through them. As short as those tabs are, the same size aluminum should work fine. If you go any thicker, you will compromise the thread bite on the clamp bolts.
Make sure the hole centers are dead nuts on both pieces, that way your Vapor will be squared up.