A sand mix is more like wet beach sand, so it doesn't flow, but will still smush some (depends on the proportion of sand to cement - you can buy it premixed - just add water - at about a 3:1 ratio; more sand means it won't smush as easily (5:1 is often used for making a shower pan slope), but the higher cement means it is a little stickier). You can put some piles of it in areas that won't interfer with the feet and smush it down. You want enough to cover a good portion of the base, but you don't need to cover it perfectly in its entirety. Put some tar paper or plastic down on the plywood, and you can cover it with some, too. The layer on the bottom keeps it from drying out to fast by the ply sucking the moisture out; the layer on the top prevents the tub from sticking (although the stuff isn't all that sticky). YOu have two goals, support the bottom and make it perfectly level.