I found an rusty 40x60cm cast iron surface plate. I cleaned is with citric acid but unfortunately there are spots where the rust made tiny holes in the metal. But a quick test with a 1µm indicator looks promising. On short distances the error is < 8µm.
I was using a magnetic base to hold the dial indicator but it has two sharp edges and is not good to slide it across the plate. Where can I found a decent surface gage for the indicator? Something like this would be nice, but that model is too expensive. Most surface gage I found have some kind of marking needle and can’t hold a dial indicator.