Creating miter joints for fabrication + thicken

Hi Team,

I am an absoloute newbie to grasshopper + rhino here and have been trying for two days to figure out how to create miter joints between extruded surfaces in gh. They need to be snug enough as each individual form is to be cnc'd individually and glued back togther again.I suspect it has something to with offsetting, scaling and then trimming... (maybe it's better to do in Rhino?)

here are my files and a picture of my attempt so far (left side offsetted in rhino)

thanks heaps in advance :)

spineFabTest.3dm

spineFabTest.gh