I feel that cabin cushions, which will not be exposed to conditions like a cockpit cushion are not as demanding as far as materials go. For cabin cushions I suggest you look at overstock.com and see the foam mattresses. I got a king 4" for 100.00 including shipping. 3 " firm foam, 1" memory. That is ALOT of foam for a 100 bucks. Makes very comfy cushions and berths. Use an electric carving knife to cut. Fabric for an indoor application need not be limited to sunbrella or other outdoor materials. Ultra leather and ultra suede (my favorite) are excellent choices. Any rugged comfortable fabric, NOT cotton or silk or rayon or linen, would be suitable. Poly comes in many very attractive upholstery fabrics with very nice hands. DO use a heavy fabric, not a light weight one. It is much more difficult to get professional looking results with a light weight fabric cover.
You might consider making a practice cover from an old sheet and then use that as pattern. Use heavy weight thread, not what you find for dress making. Again for interior cushions you do not need gortex thread.
Sailrite is a good resource for info.
a standard sewing machine in good repair will make cushions with no difficulty.
Welting is an attractive option, I suggest you purchase it ready made as making welt is challenging and you will need lots of it.
Study cushions that have zippers set into the center of the cushion side not the seam. This is what you want to do so that the cushion can be sat on comfortably on either side.
Professional upholsters use poly batting to wrap cushions to give a softer edge and feel and stuff out the corners with additional wadds of batting to make them look nice and tight.
ain't what ya do, it's the way that ya do it...