Are you sure it's not coming from underneath the mattress?

Anyway, here are some ideas:
http://www.woodroof.com/cat12_1.htm This stuff is used under cedar
shingles. You'll need a few layers though, but it's pretty cheap.

http://www.hyperventmarine.com/ This stuff works very well, but is
expensive.

You can also use plastic trellis material from Home Depot, etc. I'd
stay away from wood... just too good a substrate to grow things on!