Good ideas all around.
- I would clamp to my existing fence, but I want this fence to be quite a bit longer, and the stock Dewalt fence is... pretty flat.
- I know just the place to get scraps of countertop. I'm on it.
- I'm thinking a metal stud might be another option to clamp to.
- FWIW, I now know my own granite countertops are not flat. Even the 'flat' 1.5" thick MDF planer sled I made last year is not flat anymore. And I'm not talking about +/- 0.005. That sucker is headed for the scrap bin. The table on my saw definitely has a low spot on the left backside. I'm pretty sure the 'flat edges' on my squares are not perfect. One thing I've learned through this experience is that almost nothing is flat and to not ever assume it is. I'm definitely going to stick to wood, I think metal working would drive me nuts.
My inner scientist is screaming to build multiple fences, one w/ Jimmeim's suggestion of not clamping to anything, one with a granite tabletop scrap, one with whatever else I can come up with.