There are many things things you could use, whatever tool you can control, I personally prefer a heat gun & utility knife work great. Its the same way I cut the area out for vent kits. You get a nice clean cut. Heat the area to be cut out, don't cut out too much because you cant put it back if you do. Note: the hood heats up alot faster than the air box.
My kit came with a short piece of aluminum pipe to mold the air box & hood after heating them up. Heat bend test fit, test fit then repeat until they clear the upper steering post mount.
I used the heat gun to heat up the support for the gauge (heat it up from bottom side) so you discolor the top side or melt the decal, it is tougher than the air box. After I cut it out I fine tuned it for fit with a die grinder.