Inyo_man
Don't piss down my back and tell me it's raining.
I considered using the finish washers, but thought the lath screws would be a more inexpensive solution and work just as well.Now, if you survive or chose to go with a different source, I have used and would recommend what are known as "countersunk or finishing" washers which would work very well on attaching Coroplast. Good luck!
ABS is still on the list of possible material...thanks for the thickness spec., tirod3.
Cheers