What is typically the best bolt to hold Park BBT-22 on to square-taper spindle?
There’s actually a tool made specifically for this purpose. For example, here’s a link to the Pedro’s version: 

https://pedros.com/products/tools/pedal-crank-and-bottom-bracket/bb-socket-holder-ii/

They call it a “bottom bracket socket holder” so try that as a search term for others.