I know some people have used dremel tools. I would think a hobby type razor knife would work well. Depends somewhat on your cooler. I've had some where you could use a drill or almost any sharp pointed object to make a hole and then cut with a heavy pair of sissors. I've had others that it would have taken a dremel tool or an electric drill to get a hole and then a hacksaw to cut.
For my plastic cooler, I used a utility knife (like a box cutter type).
I drilled a largish hole first to get a good start with the knife and then just ran the knife over and over to 'score' the lines and then make the final cut.