Filters installed today in about 10 minutes. I used a long bladed utility knife and a propane torch. Literally, like a hot knife through butter. Err, plastic. I used these filters from Amazon - https://www.amazon.com/gp/product/B00UZYH2CA/?tag=gmtnation-20. I used duct tape to close it up but I had a birthday party to get to so I'll seal it up a little better soon.
Today seemed like a good day to revisit this. After initially modding and installing cabin air filters way back in July 2012 , I haven't touched them since. Today I got off my ass and decided to have a peek. As you can see they've collected a good bit of dust and dirt. I replaced them with new CF8791A filters