It's a pretty good idea, and what a lot of people use (the underlayment bit). Pretty much anything that is slightly compressible. You just want something to serve as a gasket, and make sure there is continuous contact around the rim of the tank, and to relieve pressure points.
As to putting it on the desk, that would depend on the build of the desk and the orientation of the tank. If it's an older desk, or an institutional desk (built for (older)offices, hotels, dorms, etc.) it will probably be fine, those are often overbuilt and quite sturdy. If it's a desk from a cubicle farm or for home use, I'd be fairly cautious.
You want to get the corners of the tank directly over the desk legs if you can.