If it weren't a steel dustcap, I wouldn't have even bothered. I'm hoping the washers will help spread out the load.

The cranks are currently sitting on my workbench, so I''m starting in a good position: install on a BB with some amount of torque, than try to remove. Repeat with increased torque until within spec or until failure.

I'm pretty sure the plastic cap is going to fail first, so a proper hex head screw is a must, I'd think.