Instead of cutting from the front, flip it over and try and cut the angle from there? Ah, XACTO blades probably aren't long enough. I was having that problem with my utility knife. You need the really thin knives like Andrew uses, extend it all the way out then try cutting that way. If they're like the xactos I use, the top holding the blade should unscrew

I used my blades to cut the templates out, easier on my hands then scissors