The Needswings set up seems to be one of the favorite aftermarket set ups. Other members have set up their own. Either way, if your change the set up the filters should be located just behind the grill otherwise I expect you'll experience heat soak. While there are issues with it being just behind the grill (getting wet from rain), it's the proper placement.

I think the diameter, the texture / coating and the low restriction filters in combo are all important in maximizing the benefits of the intake so if you design one yourself, those are factors that you need to consider.

Good luck.