I use a mini saw that's in a multi-tool like a Leatherman. Works perfectly as it's short and easy to control, and gives an almost exact fit. I use sandpaper or a mini file to perfect it. Once you get a routine going, it doesn't take too long. I can get through 24 arrows in about 30 mins - 1 hour using just the mini saw and file.
I do know that you can tape three hacksaw blades together, the middle one leading and the outer two backwards, and that gives you an almost perfect sized slot. I've used the three hacksaw blade method a couple of times and while it works pretty well in terms of size, I find it pretty tricky without a jig to keep the slot as straight as possible.