4.5" angle grinder with a metal cut off wheel. Can be had for cheap at your local HF!

 

It ended up being a 2 percocet job for me, I cut mine right after I got my appendix removed. May have been the only reason I was brave enough to cut it up.

Find a way to brace it while you cut it because trying to cut and hold onto it at the same time is a huge pain. Use painters tape to mark out your cut and keep the jigsaw firm against the fender to reduce the chance of it dancing.

 

tape up the fender pretty well if you're wanting to protect paint, the jigsaw vibrating on it will scratch the paint pretty bad. Go slow so you don't bend the metal up as well. But yeah, if you have the budget, get a grinder from harbor freight and some of their cheap cut-off wheels. Probably won't be the last time you'll need a grinder if you're going to be modding your bike