These conical filters (like the 9" one I'd like to put on my Jeep) have a rubber 3" or 3.5" inlet. The flex-hose from the throttle body is also rubber, so you need some kind of sleeve inside to mate them together.
What would be a good sleeve to use? I considered PVC but it's so thick-walled that I think it's probably a restriction, if anything. Really just need thin-wall steel sleeve. Hmmm..., maybe if I find an old 3" or 3.5" diameter metal can (like they'd sell brake fluid or solvent) I can cut the ends off, file the edges and use that.