More about making the shikibuton
We went to a Mattress factory store and bought a Queen size foam insert that was about 2 inches thick. It was like those camping pads you buy that has the wavy surface - but it was for a real mattress and Queen size.
We laid it out on the floor and layered COTTON batting on it until it was thick enough. The cotton batting came from a craft store, and we had to piecemeal it. It was important it be cotton so if it collects moisture it dries again. We also used a few thin old blankets to keep the batting down to the foam. Then we cut pieces of cheap upholstery fabric and made a giant 'pillowcase' for it. We sewed the pillowcase on a sewing machine but the entire open end had to be handsewn shut after the sandwich was slid in. Getting the foam/cotton sandwich into the pillowcase and FLAT proved HARD, but it could be done.
Next - we bought some thick hard foam from an upholstery shop, the half-plastic stuff. We bought it the same size as the mattress and the shop cut it into three equal pieces. This is to put under the shikibuton on the floor so you can tolerate sleeping on wooden floors with it. I planned to encase each of these pieces in more of the cheap fabric - but never actually got around to it. Each time we folded up the shikibuton up we would place the three pieces underneath it. Usually our blankets folded on top of the shikibuton covered this up - but a nice skirt could have been made for it too with just a few yards of fabric and some elastic.
It still cost us nearly 200.00 - but that saved a lot. The real benefit of the 'real' ones is that they probably clean a whole lot better than ours! In Japan people let their shikibuton dry out every once in a while, and store them during the day in closets specially built for their size.
We could also fit our 'bed' in the back of a station-wagon or pick-up truck and take it with us wherever we visited to be just as comfortable sleeping at any guest's house. However, the bed was only a few inches off the floor, so in cold weather we didn't get the benefit of the 'heat rises' thing that most people get with a bed 2-3 feet off the floor.