Yes it's easily done, pvc pipe and some rubber boots/fernco and drill some holes for sensors and such. My opinion just save your money and buy one if you really want one.
Stock setup is already a "cold-air intake" as it draws air from the fender. Throw a K&N on it and spend your money elsewhere. But it's your car at the end of the day.
I did this in back in HS on my GT for a while, primarily because I was bored and wanted to see if it worked. Wound up ditching it for a real CAI after a month or two. It worked fine I guess, in that it moved the inlet to the fenderwell and nothing broke. But it was for sure more of a PITA than it was worth. Only way making your own CAI is worth it is if you know how to work on carbon fiber and are bored.
Back in 2005 I got the urge for CAI with my Vortech S/C.
The Vortech setup mounts the air filter in the right front fender just behind the foglight.
In order to have access from outside the front of the car I moved both foglights inboard behind the lower grill. The Vortech-supplied K&N air filter is mounted just behind the foglight opening in the bumper fascia. I picked up some PVC pipe and a cap for the larger diameter pipe, steel angles, flexible aluminum clothes dryer duct, and screw-type hose clamps.
After taking many dimensions I cut the larger diameter pipe to length to fit over the K&N filter, cut a hole in the side of it and cemented a short length of the PVC pipe to the side of the larger pipe, and cemented the cap to what would be the bottom end of the assembly.
I fashioned three J-hook type clips to mount to the K&N mounting bracket to hold the filter housing in place around the filter unit. I blanked off the left floodlight opening to give the bumper a symmetrical look.
As they say "It works for me!"
PS - I do not drive the car in rainy weather, so water ingestion is not an issue.
Here's a picture up under the front bumper: