Looks really good, you did a great job with the modeling and I love your innovation that removes the need of an anvil!

I'd like to print the STL and try it for myself, to be honest, since I made my own open-source, customizable steel washer backup jig back in 2021:
n0nce's Steel Washer Backup jig (customisable)

It is much simpler than what you made, but people can get an STL for any washer size (metric, imperial, any size, any thickness).