I have a piece of wood on mine, too. I thought that was what you were supposed to do!
This copy stand is by far the worst thing about the Negative Supply gear aside form their shitty customer service. The other stuff has flaws (the first frames on 120 film are really difficult to get started. the light source mask doesn't match the leveling adjusters for the plate, etc...) but the film carriers are built solidly.
The copy stand is stock aluminum parts screwed together, it uses a junk threaded rod as opposed to a proper screw so it doesn't stay at 90 degrees, has a lot of backlash, and changes from when you adjust height to when you lock it down (because of the aforementioned sag) and even the adjusters are just cheap aluminum knobs on a set screw. I had to fuss with it to get it right, and I have an adjustable leveling base on mine so I can get the camera parallel to the film plane more easily.
If you want one more trick, now that you've got it on a piece of wood, place a mirror across the top of the film holder when you adjust the camera. If it's dead level in both directions, the center of the lens will appear dead center in the mirror. If it's not parallel it will be off center and you can easily adjust it to match. Someone here taught me that trick and it makes ALL the difference.