I get some people who inquire about getting “Down/Feather” fill for their seat cushions. First, let me say that in it's proper use, down is a wonderful material. When it is used in jackets or in comforters it is very light and keeps the warm in very well. In that use it is a great project.

Secondly, let’s address the topic of “down/feathers”. Unless you pay a lot of money for mostly down, what you get is mostly feathers (95% feathers and 5% down). It seems to me that “Down/Feathers” at that percentage is mostly a marketing gimmick. 5% down would not even be noticeable. So what you really have is feathers, so I will use that name.

I do not recommend feather filling for seat cushions (because when you sit on a cushion you put almost all of your weight on it and crush the softness out of it.)  It is almost worthless as a filling used under you to support your weight.  Although it “sounds” like it would be wonderfully soft, the reality is that you would sit right through a “soft” seat cushion and bottom out. If you fill a cushion full enough with down/feathers to make it firmer, then it becomes hard and looks “bulgy”. In addition, to use down/feathers in a seat cushion, the cushion would need to be lined. However, just lining it is not enough. If you put all the feathers in a big open cover the feathers would tend to scoot to the side as much as it could. In addition, it would not give you much support. To better the use of down in a seat cushion you would need to use a down-proof pillow ticking with several horizontal pockets: a center pocket holds a foam core (at least 1/2 the height of the cushion. it would also need to have pockets on the top and bottom for the feathers. however, when you sit on feathers they tend to move to the edges of the cushion. So, the top and bottom pockets need dividers to keep the feathers in place. However, even with all of those extra measures, the reality of what you get does not measure up to the illusion of a “down filled” cushion. Of course, no one wants to admit that they are not comfortable, thinking that they have the “luxury” of down.

The other place where down is better used is in throw pillows or in back pillows. However, again, the illusion is more profound than reality. The feathers in “down/feather-filled” back pillows usually drop down to the bottom so that is is just a big bulgy pillow. In order to make the back pillow stand upright the back cushion needs to have baffles (pockets) sewn into it (kind of like stacking 3 or 4 feather-filed tubes on top of one another. The pockets help to keep the feathers in place. This arrangement is more acceptable because as you lean back against the cushion you are only putting a limited amount of your weight against it. Many people may find this quite acceptable.

Making a little bit of a crown:

After cutting your foam to size, you can just wrap or glue about 1" bonded dacron wrap onto each side of the foam may give you the look you want. It gives a little bit of a crown.

When this is stuffed into the cushion, it will give a rounded effect. (which I use for most of my jobs.)

Getting more of a Crown:

If you want more of a crown than that (see drawing below), to give a higher crown on a foam cushion (see A in Drawing), you can start by cutting a piece of 1/2" foam about 2/3 the size of the foam (B in drawing). Be sure to taper (cut at a slant) the edges of the dacron. Then glue it to the center of the top and bottom of the foam. Then, for the layer of Dacron (C in Drawing), cut 2 pieces the same size as the foam and glue one on each side of the foam, on top of the smaller pieces of Dacron. 

Then, as papasage said (on that other board), you can cut the cushion side boxing narrower. For instance, if you are using 5 inch foam, you can also cut the boxing about 5 " wide also. After it is sewn, the boxing will be 4" wide (It will cause the top and bottom of the cushion to come down over the side a little, which, when combined with the dacron wrap, will increase the "appearance" of a higher crown.)

I hope that helps.

Best Wishes,
Stephen

Info about latex Foam

Sources for Latex Foam

Sleep Like a Bear

Latex International Talalay Latex Mattress Toppers

Sources for Adhesives for Latex Foam

Most upholstery supplies will handle foam adhesives.

Foam for Upholstery

    You use a much softer foam on the backrest than on a seat because, when you sit on foam you are putting much more weight on the seat foam. By contrast, when you lean against the backrest, you put very little weight against it. (Remember, most of you weight is going to the seat foam.)
     Most seat foam has a firmness rating of aprox. 26 lbs to 30 lbs (soft), 30 lbs to 40 lbs (medium), 40 lbs to 48 lbs (Firm) and 49 lbs to 55 lbs (Extra Firm)
     Conversely, firmness ratings for backrests run about 15 lbs to 20, very soft 20 lbs to 24 (medium-soft)
soft foarm for backs
     So, when you choose a back foam

Foam is Rated by weight and density