I would be very careful with overhandling the Robbie faceplate.
I recall a number of folks saying that the paint comes off too easily.
My MRM cowl had the same issue as yours, the Robbie plate would not sit flush on the one side.
Instead of messing with the faceplate, I carefully carved out the inner mouth area of the cowl
a little at a time until I got the faceplate to sit good enough in the cowl.
Main issue with mine was that the plate did not sit deep enough in the mouth area.
So, you may need to carve that mouth area deeper.
It was over a year ago, but I think all I used was a small blade/knife to carve the cowl.
Good luck, bud!