Hi Charter,
There is more information about choosing a topper in post #2 here and the topper guidelines to which it links. If you would be trying to stay within the theme of a firmer product, perhaps something along the lines of a 2" Dunlop “medium” topper might be worthy of investigation.
Sleeping on what generally amounts to wooden or grass matts is more common, and traditional, in Asia. I was reading an article about this a few months ago and the researchers were speculating as to why these firmer products (and firmer mattresses in general) were more popular in certain areas of the world. Some of the research led them to speculate that there has developed a difference over generations in the amount of lordosis (forward curvature) of the lumbar region of the people in these areas, and with a less pronounce curve, the people in their research were better able to tolerate firmer surfaces. Some of the other research speculated that it might be simply this is what the population is familiar with, so this is what they use.
Matts made of bamboo are a common material. There are some other matts you may see in stores that look like a collection of small branches strung together, yet there are different diameters at different prices. Tatami matts are popular for sleeping and for flooring, and these are made from woven rush grass around a straw core. On top of these a shikibuton is often placed for sleeping, and this is placed in a closet or a cabinet during the day.
Many people do prefer this more traditional sleeping arrangement, but it is a firmer surface with little, if any, progressive resistance, so quite different from what is common here in North America. In the end, like so many things, it comes down to your own personal comfort preference.
Phoenix