Can you build a cold air intake?

I don't want to but my dad knows a guy who is a mechanic who can make me a cold air intake. Would this deliver the same performance as a k&n Cai or other high performance ones that promise big gains I have an 04 Mazda 6 3.0l v6 would it be just as good. Also cpe makes one that says it delivers up to 20hp and 20 ft lbs of torque would the one he builds be as good or do they do something different to achieve such gains?