The only problem is there's a privacy panel behind the right side of the desk, and it's structural, so I can't remove it or cut a hole in it...and the cables sticking out of the back of the computer make it protrude from under the desk by about 4". So I can't get my desk chair in the right way.
So! After I've had dinner, I'm going to move the CPU back to the left side of the desk, where it was originally lo these many years ago, and the only problem I'll have then is remembering that the trash can is now on my right rather than my left.
But as predicted, the materials cost less than $20 and, in fact, were barely more than $10 with tax. Two pieces of particle board shelving, 36" long, about $6 for the 10" wide one and a bit less than $4 for the 8" wide one. Cut 'em to 18" lengths with the circular saw. Four drywall screws (already on hand) and a few minutes' work with my cordless drill and a T-square.
And yeah, the fan noise is a lot less bothersome with the computer back under the desk.
(Pics after dinner and probably after choir practice.)
UPDATE: Ehh, maybe pics on Tuesday or whenever the hey I get around to it. Heh.