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
Just insert the sleeping bags (without folding and zipping up each of them in order to get the maximum width of 2x33" = 66") into the mattress cover one on top of the other. I found that I can easily creep into the cover myself once all the sleeping bags have been inserted in order to smoothen out the contents inside and make sure that they are lying flat on top of each other.
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/