You can do a couple of other things to build a cove that are relatively cost effective.
1. As Dennis had stated, tape plastic (vapor barrier) about 6" up the wall, letting it hang down and out on the floor. If there are gaps, make sure to push the plastic into them using a putty knife. You can build a sand cove on this plastic, the sand will then fill the space where you pushed the plastic in and it likely won't ever wash out.
2. As your hubby allready suggested, make the cove out of left over styrofoam, it will be a bit tedious to get a good fit for the bends, but once you make 1 or 2 templates, things should progress nicely - nothing wrong with using the styrofoam.