Hey Eric! You'll hopefully be getting a set of submission pics for me soon. :-D
I trimmed mine to about half an inch thick, just going anecdotally based on guesstimates from other peoples' build threads and what I could find online. Mine came out a bit sloped - it's thicker towards the bottom and thins out on top. I initially trimmed the piece I got from my ATA kit, which was hollow, then filled it with two layers of foam board and cut an ABS back for it. Can take pics if you'd like. It's not the slickest knee plate out there, but it looks banged up and dirty.
Some more experienced members here may have a more precise answer, though. Best of luck! Hope to meet you at a troop soon.