Re: Boat cushions
My canvas guy made cushions for us out of 1.5 inch closed cell foam with an outdoor nylon(?) type fabric that dries out fast, doesn't slide and is comfortable. He sewed tabs into the corners so that if I found the cushions were moving about adding snaps would be a breeze.
I had looked at bottom sliders and didn't care for them. They were more than what my canvas store charged and I didn't think they wouldn't hold up well in usage.