@ai6yr
For the pitch, you need to screw the nut onto a bolt on the gauge for at least three turns, or vice versa.
So many bolts are machined with a taper or even a rounded end. One turn appears to fit, but the wrong pitch will get stuck soon enough.
Or use a handy calculator Like this. Count threads per inch first.
https://www.omnicalculator.com/construction/thread-pitch
Or try a chart 🤞 🍀
Oh yeah, the wood should grip the square on a coach bolt if you drill the holes just big enough to take the shaft.
@ai6yr side quest: I grew up hearing only the term “Jerry-rigged”, and M-W says they’re both valid although emphasis is slightly different!
https://www.merriam-webster.com/grammar/jerry-built-vs-jury-rigged-vs-jerry-rigged-usage-history