[Sinkoumn]

Can anyone point me in the right direction for creating a concave on a surface. I am looking to make contours on the top of a slab that looks like the top of a skateboard.

Here is the board that I wanted to add concave to, I would want the parrellell ends to have the same concave, but the kicktails are going to have a little more height than the sides.

Any help would be greatly appreciated!