I'm still thinking about making a open cone one myself. The new intake design is much easier to use just a filter and an aluminum bend. Here are some shots somebody took at e90post:
I opened up the intake box and it's a shallow box that gets deeper at the far end. Given that there's no throttle body there, you only need to add a 90 degree bend to that single inlet and add the filter at the end. My only concern is to come up with a custom support to hold the filter and the air scoop in place..