Matte protector I cut down was from this
Supershieldz iPhone 6 Plus / 6S Plus Front + Back Anti-Glare & Anti-Fingerprint (Matte) Screen Protector Shield -Lifetime Replacements Warranty [3 Front And 3 Back] - Retail Packaging
https://www.amazon.com/dp/B00P4B0VO...ag=hawk-future-20&ascsubtag=UUimUvbUpU2892122
I use to use the back skin in that on back of 6S+ with CrashGuard. I had the fronts and just cut one down with scissors. Had I used a device as you describe I could've certainly got it "straight" and it would look better. Can hardly notice I didn't cut exactly perfectly though.