I'm in the same boat as ChaCha -- (you've got an X-1, right ?) -- anyway, I'd suggest preparing a durable _mount_ for whatever you decide to try, and then try a few different things.
as far as noise absorbtion goes, I would think that mass is more important for absorbing lower frequency noise.
fan noise is higher frequency and I'd think that mitigating reflection of that fan noise is at least as important as the absorbing material itself.
so. I also have an X1 directly above my head, and between bookcases at that -- which backs up to a wall. a 3-sided tunnel, only open at the front.
I'll probably use lightweight plywood mounted horizontally below the projector, w/ some foam above that to kill reflected fan noise.
for appearance, I'll do a nice funish on the underside, and put a lip on the front to hide the foam ...
some of that noise comes out the front of the projector and I don't know yet what to do w/ that ...
ymmv,
Mark