When I did my conversion on a D80, I just used a hardware-store glass cutter, and a table.
I bought a 72 (I think) mm Hoya R72, removed the filter glass from the threaded holder, and cut it with a cheap roller glass-cutter.
I laid the stock filter-stack on top of the hoya filter to get a idea of where to cut.
On the other hand, I bought the camera specifically to convert it to IR, and have little sense of electrical caution.
I then proceeded to completely tear apart the AF sensor assembly and metering sensor, to remove the IR filter in those devices too. However, that's generally not needed, and is an absolutely gargantuan pain in the ass (faaaaaaar more then just changing the filter on the sensor).
I also drilled new holes in the cover-plate for the bottom of the camera, so I can access the AF sensor calibration screws with the camera completely assembled.
It still works!
(Except the built-in pop-up flash, and I'm ~90% sure that's just because I forgot to connect something. I'm just too lazy to take the whole camera apart again just for that. IR photography works best using natural/sunlight anyways).