I never sit on the furniture. I have a little square floor cushion & I sit hunched in front of the fireplace on that.
Recently I came into some rather large pieces of apholstery fabric which is way too good to cut up. I've been wondering about making floor cushions but really couldn't be bothered with the measuring & triple checking associated with making a box cushion cover fit an insert perfectly. Secondly, you hardly ever get lovely long zips at the Op Shop.
Yesterday I had a little flash of genius. I had a bag of polystyrene beans left over from when the kids had those kiddy bean bags, so I filled the cushion with those. Best of all I only had to use a little zip (which are in plentiful Op Shop supply) & I had everything on hand to make it. It's around 60cm square & super comfy.