Normally I'd suggest cardboard and tape to mock one up, but you're most likely going to need a drop base air cleaner - and that could be difficult to fabricate out of cardboard.
I went into one of the locally-owned, small speed shops which I frequent, and they let me park out front and try different models to see what would work. As it turned out, they didn't have anything. I ended up doing a whole bunch of measurements which indicated I could fit a 14" x 3" air cleaner using a 1 1/4" drop base (
http://www.summitracing.com/int/part...3549/overview/).
That was all well and good, but the 460 in my car sits too high and the top of the air cleaner still made contact with the hood. The 460 is most likely higher in my car than your FE, so a combo like this may work for you.
There are a few other Kirkham owners on CC with FE engines, so many have faced and solved this issue. I'd suggest searching the Kirkham forum (
Kirkham Motorsports - Club Cobra) for "air cleaner" (quotes included), as there are multiple threads / posts where the issue of which air cleaner will work has already been discussed. If none of those are exactly what you're looking for you can always start a new thread there.
It may also be worth a cruise over to a friendly speed shop with your Cobra to try out a few combinations to figure out what will work for YOUR car.