I balked at the cost (and appearance) of the KEP air filter box on my second conversion and fabricated my own setup.  I closed off the area behind the drivers rear wheel well, creating a cold air box, and installed an aftermarket cone style air filter therein. 
Performance is good with the aftermarket air filter but I consider the noise level to be unacceptable.  If you stand by the d pillar air intake on my van while it's sitting at an idle, you'll be craning your neck, trying to look in through the window, to see who is torturing a howling poodle inside!!!  The filter gives off a low droning sound at around 800rpm.  At top acceleration, the filter gives off a raw sound which is quite loud.  Eric Didier's dad's (sorry sir, I've forgotten your name) converted vanagon also uses an open style aftermarket air filter and for whatever reason is not near as noisy as mine.  Might be the fact that I have mine entombed in the metal box. 
The noise level is not at all what I'd call objectionable to most folks.  Because I'm after more of a stealth volume level, I will prob change it out later and go with the KEP horn or continue modification and enclosure of the aftermarket filter until I get it quieted down. 
 
All in all, I feel that the KEP setup is a bit too costly but is a fine piece of engineering and design.  I also believe that for the price it should come with a bracket to secure the filter to the wheel well.  It's not hard to build your own but why should you have to?  Anyway, with the money I've spent on materials, and the value of my time at a nickel an hour (that's my going rate) I have likely spent enough to get KEPs unit by now and will likely do so next time around. 
 
Thanks,
Brent

Billy and company:
The only reason I didn't go with the stock suby air filter is because I had decided to try to get what I thought was going to be a quiet setup and stuff the aftermarket air filter in the area behind the taillight, closed off from the engine bay by a fabricated partition.  I was very surprised at the drone noise level and even more surprised today when I moved the aftermarket filter back into the engine bay and heard how much quieter it is.  Go figure. 
I believe you have the right idea with the stock filter setup though.  You're smog legal and your costs were much lower than mine.



Picks of Air Cleaner Setups

Kep Horn
Suby Stock Box
Aftermarket Filter