When I made the fence for my sled, I first milled a piece of 12/4 red oak longer than the fence, as flat as I could get it. Then I glued up the two 3/4 plywood fence pieces and clamped them to the piece of oak. The two plywood pieces had some bow, and I arranged them so they both bowed out in the middle, in opposite directions, so there was a fair gap between them until it was all clamped. I left it clamped for 2 days. It came out flat and has stayed flat. The advantage of using the piece of oak is that you can put many clamps all around the fence pieces. It's harder to clamp something like that as well to a bench top or table saw top.
--I had my patience tested. I'm negative--