Wow. I was looking forward to a bunch of replies to this thread since I'm building machine number 2 right now. Maybe it will get some legs.
What I used on my first machine was lanes of extrusion. It usually has a MDF spoilboard sitting on it, though. I don't think I have pictures of it without the spoilboard. I'll check.
For my next one, I'm thinking aluminum plate with a grid of threaded holes.