I recommend that you (and any other customizer who doesn't have one yet) spring for a dremel! Now that I use one I can't imagine ever making figures without one! It would probably have saved you some of that cutting on the coat.
I actually own 3 Dremel rotary tools, a plug-in 2 speed, a rechargable variable speed & a micro that runs on 4 rechargable AA batteries... this just required some good ol' fashioned, primitive, precision w/ my X-acto
--- It really didn't take too long... I placed the coat on the torso & then carefully carved the shape, using the shoulder armor as a "guide fence" --- I just really took my time, b/c I only had one shot & it had to line up perfectly --- gluing it in place was a bit of a challenge as well... OH! & I had to trim down where the lapels meet the armor for a snugger fit (it was warping & not lying as flat as I wanted...
"I'm Mighty Tighty Whitey & I'm smugglin' plums..." --- obscure Bloodhound Gang reference - LOL