For my last van, I got tired of the flimsy regular seat covers that you can buy in stores.
So I ordered them in an online (French) shop specialized in professional truck seat covers. The difference is incredible: thick, extra strong, extra durable, stays in place. Well worth the price (around 120 AUD for one).
I also have the neoprene temporary seat cover that surf brands sells, such as
www.billabong.fr/3mm-couvre-si%C3%A8ge-3mm-pour-homme-L4AS01BIF8.html?color=0019 handy and waterproof enough for a quick change of spot mid-session. Try to dry yourself as much as possible with a towel before, salt water on the floor will make things rust quite fast. I would not use them for regular use however: they are not fixed, and you would sweat a lot.