I bought this Honda CMM touch up paint for my EX, but now I don't need it since my Si is SOP. I'll give this to whoever posts a pic of their CMM car here first. You will just have to Paypal $4 for shipping in the US.
Just build a custom fiberglass box, install an amp and call it a day. I've never heard a free air sub that produced quality bass.
Box cost me about $200 in materials. Sub was $150ish and amp was $180ish.
Not a cheap route, but very rewarding and oh so stealth.