Hey @TroyD I don’t mount it to wood.
Again many people don’t need this level of accuracy and I also have other posts coming that even if you aren’t mounted to a bed that isn’t 100% accurate will show you how you can improve accuracy.
I have seen another picture of a build on the forum that better demonstrates this. It places a metal bed on top of soil, note they didn’t fill the bed with soil, it just sits on top. I searched for this pic but couldn’t find it sorry. If someone can post a link to it that’d be much appreciated.
The original tip I was given was to use angle \ L shaped rail, rather than linear rail and then mount that to wood using cheap brackets, everything bought at the hardware store.
Another option is to mount the farmbot linear rails on spikes, then line up pieces of string that are perfectly perpendicular, hammer the spikes in along the lines and do a 90 degree + spirit level check. Note the join in the linear rail may need to be enforced to ensure they stay straight. I have never tested this option. I think laying the metal bed on top of soil is probably easier.
Hope that helps.