Ok, granite surface table, grade 0, sine table and engineers gauge blocks. For 90 degrees there are vertical standards. Everything else is simply nearly accurate. A temperature and humidity controlled room would also be very helpful. Now, I’m sure you think I’m being ironic or sarcastic, but I’m not. If you after 10 micron or better accuracy over 150mm you need to dig deep into your pockets and accept that a square, or anything else simply isn’t going to make the grade, anything aluminium is worse than useless. It needs to be hardened, ground and looked after with it never being bumped, dropped or knocked.
Once assembled, calibrated back to national standards to a level about 10 times better than what you want to measure you need to carry out initially a type 1 and then a type 2 R&R study.
If It’s for metalwork, then most company’s never mind hobby shops don’t work to this level of precision. If it’s woodwork, your wasting your time, wood work machine are not built to this level of prevision.