A jig saw would get better cuts.
If it were me I would get a piece of poster board and a compass, Measure the diameter of the pot and make a template with the correct size hole on the poster board. You can size it up with scissors and a mistake is much cheaper. Once you have your template you can use it for all the future builds and not have to measure again. Use a sharpie to trace where you want to cut and follow it with the jig saw. (You will also need a drill to start the cut which you can also find for inexpensive.)
No guessing required!
Here is the type of jig saw I'm talking about:
http://www.sears.com/shc/s/p_10153_...01&srccode=cii_17588969&cpncode=31-29288641-2
I have a fancy expensive one and a few cheap ones and to be honest unless you're trying to do something really intricate or cutting into hardwood the inexpensive ones are fine.
When I start building cages I will do a step by step with all tooling details and how to, I've been working with wood since I could walk pretty much.