I've read around and some people have said to buy one (I don't want to pay the money for a comptech icebox..), so I'm wondering if there are any guides/instructions on how to do this.

Meanwhile, on my stock intake, there's already a large plastic box covering the filter - is this a heatbox, too? And if so, would it be wise just to use the stock heatbox with a SRI or should I make a better one?