I drilled out this plate on my drill press. However, when I attempted to file straight the slot between the drilled ends of the slot, I hadn't noticed that the edges of the file had 90 deg/cornered the edges of the slot. From what I gather, this isn't ideal, if for no other reason than not allowing any corner on a plane part (due to introduction of stress risers for cracking/tearing). I emailed Zenith to see what they say if I should just order new plastic/nylon plates and rebuild. I'm assuming so at this point. While waiting I'll just continue on the flap build and get back to it later.

Update 10 Feb 2020 - Zenith got back to me. They said that it probably isn't a big deal and that as long as i cleaned up the corners so they are rounded (with a small round file) I should probably be ok.
After thinking about it, I opted to go ahead and buy new ones and try again. This because I hadn't cut out the matching rib slot yet and didn't want to have the mistake compound itself (even though it still probably doesn't matter). New parts on order.