First off, for these CV type carbs, you'll want to use UNI straight foam pods, not the K&N pleated type. The UNI you want for the BS38's is the UP4200. It is 4" long and is the longest you can fit in front of the battery box. It has a 2" diameter inlet and that fits the BS38's like a glove .....
And don't bother getting the special filter oil they make for foam filters. It's too thick and may clog these pods. Just use a 50-50 mix of gas and motor oil .....