BHAF
jeff, the BHAF inserts directly into the airbox end of the air hose--no adapter needed. i've heard of some folks using a short piece of 4" PVC to keep the mouth of the filter from collapsing. no problem with the NAPA i'm using so far.
i've heard prices quoted from $30-$50 for different of the filters, depending on the source. mine from NAPA cost me about $40.