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 https://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.
Step 2: Insert All the Sleeping Bags Into the Mattress Cover
Step 3: Voila' - You Have Created Your Inexpensive, Portable, Lightweight, Out-Of-The-Box, Machine-Washable Mattress Substitute
Finally, just zip up the mattress cover, and voilà - you've created your own portable, lightweight, inexpensive machine-washable mattress-substitute for around $84 using off-the-shelf materials easily available at your neighborhood stores. (OK - the vinyl mattress cover may not be machine washable - but if you choose to use a cotton mattress cover, it wouldn't be an issue).
1) Obviously, you may use as many or as few sleeping bags as you like - as long as they fit inside one mattress cover. You may even use more than one of these mattress substitutes - all depending on what gives you the right amount of comfort.
2) I use my own improvised out-of-the-box platform bed (which again, is just as portable, lightweight and inexpensive with no tools or special workmanship required for assembly) to go with this off-the-shelf mattress substitute - details are at https://www.instructables.com/id/Portable-Improvised-Lightweight-DIY-Substitute-f/