I thought of this inexpensive out-of-the-box DIY solution using easily available standard components (a few sleeping bags and a zippered mattress cover) that requires no tools or any kind of workmanship - which I am using as a mattress substitute - and it's working great.
The platform bed I am using to go with it is also similarly improvised, out-of-the-box, portable, inexpensive and lightweight, details of which are available at http://www.instructables.com/id/Portable-Improvised-Lightweight-DIY-Substitute-f/
Step 1: Buy Eight Inexpensive Sleeping Bags and One Zippered Mattress Cover
The only other item I am using is a queen-size zippered vinyl mattress cover I found at Target for $11.99 (as of 05/26/10) - http://www.target.com/Vinyl-Zippered-White-Mattress-Cover/dp/B000BFK7R0 . You could just as well use a zippered cotton mattress cover for a comparable price.