re: Vacuum clamps?

I've seen the ones that Rockler sells but have not tried them.  I picked up a couple of vacuum chucks and a pump on Craigslist a couple years ago for finishing the bottoms of bowls on my lathe.  Works great for that so I can imagine that for hold downs it would work well too.  My only concern about the Rockler ones is that they are pretty small.  My understanding is that the larger the area of contact, the greater the holding force but maybe that is not an issue at maximum vacuum.