Here's how I made my rocker table with concave:
Cut 3 - 6 foot long 2x4s with a band saw in the shape of the rocker that you want (cut rocker.jpg). Make two of them identical. Make the third one 3-4 mm thinner. Keep the part of 2x4 that you cut away because this will become the top of the mold. Make sure to mark them before you cut them so that you can match them up again when glueing.
Now attach them together with cross members and wood screws. Put the thinnest one in the middle. (Crossmembers.jpg)
This is a picture of the cut away part of the rocker. (top of mold.jpg)
Make sure that your cross members are straight and of the same thickness.
Lay the cross members form on the driveway or another flat surface with the cross members down. Lay the sheets of plywood on top and glue together. I use gorilla glue. I also used 3x3mm Baltic birch at the suggestion of Trent. (not anonymous).
Now match up the cut away portions of the 6' - 2x4s on top of the freshly glued plywood with the rocker table. Use the marks that you made to line them up.
Next add more cross members on top , perpendicular to the 6' forms. Now clamp the heck out of it. Clamp at every cross member and in between them if you have enough clamps.
Here's a picture of the rocker (rocker.jpg)
and of the camber (camber.jpg)
In all it cost me about $12.