If anyone has the tuning kit for the N14 Cooper S, or a N14 JCW with the Tuning Kit airbox swap, a good option for replacing the paper filter to a reusable cone filter is the aFe 24-35507 MagnumFLOW A/F (Pro 5R). I had tried fitting the K&N RU-3130 filter that everyone recommends on NAM, but the filter flange didn't fit well against the hose inside the airbox. The aFe has a very pliable flange with an inner diameter of 3.5" and when tightening the clamp, fits it snug against the airbox hose. The other dimensions on the filter make it a perfect fit inside the box when turning the holder piece at the end of the box upside down. Best part is it's half the price of the Madness Motorworks filter, about 25% cheaper than the K&N and completely re-useable unlike the paper filters.