I think you would get a bit of a gain even from just slapping together some corragated sewer pipe on. There are a lot of opinions around about airflow, swirling or smoothing, etc. Any gains you get from this are going to be small. I say just make it work and try to keep the inside of the contraption you come up with clean and fairly smooth.
You can get adapters for the top of the weber to various diameter round holes. That and an elbow of some kind would get you into the cold air intake kit you already have. I think
http://www.carbs.net carries a bunch under weber air filter adapters.
I was figuring to do something like this on mine someday soon for much the same reason. I actually took the weber off my old truck because I got tired of listening to it breathe. The only thing that sucks about a weber is the sucking sound of more air screaming down its throat.

Fuel Injected Troopers run a tube over to the drivers side fender just behind the headlight to where the filter is. There is an airbox around it, and air feeds in from another tube across the front behind the grill. I figured I'd do something similar minus the grill pickup. It would be nice to have things sealed with an option to add a snorkle later one. 78 and later LUVs already have that air feed over by the head light, so that would help you get colder air in even without some sort of airbox to keep out engine bay air.