I wouldn't waste time and money trying to clamp the bastards! Those shields are just thin aluminum and even the slightest misadjustment or bend will cause problems, not to mention broken mounts, etc.
I just took some tin snips to mine over two years ago...they had more rattles than a baby toy! I left the ones around the cats, but all the ones wrapped around the pipes came off. I wouldn't remove the ones around the cats or the flat shields attached to bodywork. Just remove the ones that actually wrap around the pipes.