It's easy to measure them. Most upholsterers will know what to do with even raw numbers (such as when you provide them with only the flat-bunk dimensions). The best help is to make a set of templates using kraft paper, capturing the dimensions of the flat bunk top (use Sharpie marker), and then either folding more paper up the side of the hull to represent the curvature of the cushion's thickness or just interpolating it and adding the numbers to the flat-bunk measurements. Anyone experienced in doing boat berths will understand that.
If at all possible try to get closed-cell foam. Also, there is an absorbent matting that can be bonded or glued to the underside to keep the foam from sopping up humidity from the plywood bunk top. Do not doubt that it will sweat horribly, adding to cabin humidity, mildew, bad odor and a propensity to rot.
When you go about making new foam, keep in mind that no one likes to sleep on any less than 4"-thick cushions. I think I will do my new ones in 5". Stiffness counts too-- I do not think you can have them too stiff, since you can always add a thin layer of softer foam to the top and make them more liveable. The 4" I sleep on now is too thin when I lean on an elbow or kneel on it; and it's just about impossible to make 'too-soft' any stiffer.
I maintain that the simplest upholstery is the best for smallish yachts. Too much 'fluff' begins to make the boat look like it's trying too hard. Be sure to add the batting to upper inside corners (where people will sit) to keep the edges from being too blockish. Other than this, straight and simple will be cheapest and easiest-- and will look most appropriate in a small necessarily cluttered cabin.
As far as fabric goes, Sunbrella would be unnecessary and prohibitively expensive for down below. You are NOT restricted to fabric marketed as 'boat upholstery'. Any cotton 'sailcloth' fabric or anything viable for heavy-duty upholstery will suffice.. Natural fibers are preferable to polyesters. 'Too-thin' wears too quickly and 'too-thick' is rough to sleep on.
I found some denim in a popular fabric shop around here and can't wait to get going with it. Some of it comes with little embroidered crabs or shells or lighthouses, and I am making a throw pillow out of each variant.