One Corner monitor $99
https://www.grainger.ca/en/product/CONCRETE-CORNER-CRACK-MONITOR/p/WWG38YZ69
CRACKMON 4020A Heavy-Duty Building Foundation Crack Monitor for Concrete, Masonry and Stucco Buildera Manufacturer
Two of them $20 each
https://www.amazon.ca/CRACKMON-4020A-Heavy-Duty-Building-Foundation/dp/B00QWFYA7G