I just sit mine on my keyboard shelf. It's hidden from co-workers and within easy reach if needed. If I was going to hide it under the desk like that I'd just use a couple of pieces of neoprin and a good stapler or, as others have said, wood screws. Cheap and easy.
I purchased a hard plastic in the pants holster, cut off the pants clip part and then screwed the gun part of the holster to the bottom of the desk. I had to counter sink the screws into the holster so they wouldn't scratch the gun.
I saw an article years ago for a desk rig, which was basically a plastic rod with a good 4 screw mount. The rod stuck out toward you under the desk and you would slide the barrel over the rod so that the grip was ready to grab.