I made mine out of timber. I constructed the frame and then used lofting chipboard to make the surface. Each piece was checked for level and where required thin pieces of ply were used to get it absolutely correct. I found that by the end of the build there was a very slight sag in the middle but it did not affect the chassis itself. It does seem like even the sturdiest of constructions will settle over time. My chassis was made in about 2-3 weeks so the settling was quite minimal. If I was doing it again I would use dexion or similar and would put in more vertical supports. Having said that my chassis appears to have come out as close to spot on as I can measure with tools that I have available to me. a mm here or there is going to make very little difference, I would say even 3mm would be nothing to worry about. Any more than that and I would be looking to redo it, others may have a different view.
Hope that helps a bit.
Enoch
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