Hi plusfuture,
There are some foundation suggestions in post #1 here but a good foundation that would provide suitable support for a shikibuton will almost certainly cost significantly more than the Ikea foundation that you were considering. With a sleeping system as thin as a shikibuton I would suggest closely spaced slats that are no more than 3" apart and closer yet would be better to prevent you from feeling the individual slats and to prevent the shikibuton from sagging into the gaps.
If the height of a foundation isn’t important to you then you could also use something like the bed rug here (or a tatami mat) directly on the floor.
Foam inserts would be softer than densely packed cotton. Polyfoam will also soften over time while cotton will become firmer over time as it packs down and compresses.
A sleeping system (such as a mattress or a futon) is only as good as its construction and the type and quality of the materials inside it regardless of the name of the manufacturer on the label. Different grades of cotton with different fiber lengths would be more or less costly (and organic cotton would be more costly than non organic cotton) and the cost of the materials would also depend on how the cotton was cleaned, processed, and made into batts (vs raw cotton which would be lumpy), how it was layered and tufted (to keep the cotton from shifting and getting lumpy) and the type and quality of the fabric that enclosed it. Having said that the “raw material” or “commodity” cost of the materials in a sleeping system aren’t always reflected in the price of a mattress and different manufacturers can also sell very similar products for different prices (see post #14 here).
Gold Bond is a reputable manufacturer that has been in business for over 100 years and are one of the oldest mattress/futon manufacturers in the country (see post #3 here).
Polyester is a synthetic fiber made from petrochemicals that is used for batting (like cotton) or to make fabrics (also like cotton). It is very cheap so it is often used to replace natural fibers to lower the cost of a product. It is completely different from polyurethane foam which is a foam not a fiber. Like all fibers polyester will become firmer as it packs down over time while foam will get softer. In most cases cotton and polyester fibers will be blended and the greater the percentage of polyester the lower the cost would generally be. Other types of natural fibers (such as wool) will be even more costly than cotton.
Polyester doesn’t absorb moisture like natural fibers and natural fibers are much better for temperature regulation than synthetic fibers. Natural fibers are also more durable and don’t break down as easily as most synthetic fibers as well.
I can’t speak to the comfort or support of a futon or mattress for any particular person (especially if I haven’t slept on it myself) but there is some general information about about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel”.
You are also the only one that can feel what you feel on a specific mattress/futon and there are also too many unknowns, variables, and personal preferences involved to use a formula or for anyone to be able to predict or make a specific suggestion or recommendation about which mattress/futon or combination of materials and components would be the best “match” for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) based on specs (either yours or a mattress) or “theory at a distance” that can possibly be more accurate than your own careful testing or personal sleeping experience … hopefully using the testing guidelines in the tutorial post (see mattress firmness/comfort levels in post #2 here).
There is also more information in post #2 here about the different ways to choose a mattress/futon (either locally or online) that is the best “match” for you in terms of PPP that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for that are involved in each of them.
When you can’t test a mattress/futon in person then the most reliable source of guidance is always a more detailed phone conversation with a knowledgeable and experienced retailer or manufacturer that has your best interests at heart and who can help “talk you through” the specifics of their mattresses/futons and the options they have available that may be the best “match” for you based on the information you provide them, any local testing you have done or mattresses/futons you have slept on and liked that they are familiar with, and the “averages” of other customers that are similar to you (although choosing a thin and firm futon would put you outside of the averages that most people would be comfortable with). They will know more about “matching” their specific mattress designs to different body types, sleeping positions, and preferences (or to other mattresses that they are familiar with) than anyone else.
Phoenix