Figured out a relatively inexpensive and easy way to make a cold air intake that should work on Mustangs, Cougars, Fairlanes, etc. You just need to have a hood scoop like the non shaker type scoop that came on Mustangs, Fairlanes, and Cougars and a hole in the hood under the scoop.
You can use a regular air cleaner from just about any car that has a diameter of about 17 3/8" or so. I just got a cheap one from a 302 2 barrel.
The air cleaner needs to be modified a bit so the ram air seal can properly sit on air cleaner base. The top of the air cleaner wall is too thin and lacks a large enough lip for the seal to hook on to. The air cleaner lid is a bit larger diameter and would provide a wider shelf for the seal to sit on and the lip on the outside diameter of the lid would provide a place for the seal to lock on to.
I removed the rubber seal on the underside of the air cleaner lid and cut off the outer 1/2" of the lid that held the rubber seal using a scroll saw. Here's a photo of the lid with the outer edge cut. The cut off section of the lid just needed to have the sharp edge filed and bent down a bit to be parallel to the air cleaner wall so they could be welded together later on.
The cut off section of the lid sitting on the air cleaner base:
This is a repro ram air seal for a 71-73 Mustang that is sold at a lot of Mustang parts vendors for about $75. The photo shows the under side of the seal and the underside of the section from the air cleaner lid:
The lid section sitting in the seal:
Photo of the section of lid with seal sitting on the air cleaner base. The lid section isn't yet welded to the base but it still sits on there pretty well. I had a cover from another air cleaner for the hole in the base for the removed snorkel but it would be easy to just screw on a piece of sheet metal or aluminum to cover the hole. Same for the other small holes in the air cleaner for smog junk.
The 14" diameter air filter is about 2 1/2" high in the photo. A 3" tall filter would not let the hood close. This is in a '69 Mustang with a Victor intake on an FE. How tall of an air filter you could use would depend on your combination of car, engine, and intake. If you drive the car a lot and are concerned about rain/water, you could probably just drill a hole in the lowest point of the air cleaner base and even put a nipple and drain hose on and run a metal air filter lid.