Big NO to just plain shellac or polyurethane. Both are going to be too slick! Sure, for the sides and bottom, I would go polyurethane, but I would want an epoxy with some grit in it for the top where people are going to actually have their feet. You could add grit to the polyurethane too, but it won't last as long as epoxy.

If it was me, I would coat the whole thing with clear epoxy and then finish the top with grit added epoxy. You can buy bags of grit on Amazon to add to clear epoxy or polyurethane. http://www.amazon.com/40202-Non-Skid...rds=epoxy+grit