Hmm, one thing the other posts might not have addressed is what kind of wood are you using? Are you using construction 1x4s? If you use wood that is already straighter and square , it reduces how much work you’ll need to get things working.
Note also that wood is flexible to a degree, so you can with the correct joints and clamping get things to fit.
Something to consider is to modify your plans to consider small errors. What kind of joints are you able to produce? Can you cut straight lines at a consistent depth?
Btw if you need the miter saw to cut a larger angle you can use an existing known angle as an insert.