cutting port opening newb

I'm just curious, is there an easy way to make a very precise cutout for the opening for a slot port on the baffle?  I'm very inexperienced at woodwork and I just completed my first ported enclosure and the cutout for my slot port was about 1/16 to 1/8 of an inch smaller than the actual port of the enclosure.

So, basically, I had to use a jigsaw to try and take off a very small amount of wood which left me with a rather crooked cut and quite a bit of filing to do to make the sides match up.  I'm just praying there's a simpler way that I missed somewhere.....

Thanks in advance!

you can use a router and a bit attachment such as this:

http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10051&langId=-1&catalogId=10053&productid=100076450&N=10000003+90401+524404

Of course there is more than one way to skin a cat.... but they are all pretty messy.