I have been looking for a precision square to align may machines and also my combination squares. I found the PEC tools solid square on eBay. Un like most other low cost squares, it comes with a squareness specification : 0.0006”/6”. I checked it using a dial indicator, it indeed looks to be within that accuracy.
You can find this square for sale here.
Made in the USA, High precision all for $ 26! The only downside (if you could call it that) is that it does not come in a case. But that is just an excuse to make a custom wood box! My unit is going to live in the house only to be pulled out when I want to check my working tools. However for $ 26 it is inexpensive enough to use on a daily basis.