Strips instead of rails.

To hold the pailings in position, I've got a solid 50mm x 65mm rail at the top and 19mm x 65mm hardwood strips each side of the fence at the bottom and the middle.

These strips are kiln dried spotted gum and hold their shape very well with little warping and bending. They also dressed all round and look great.

The bottom strip sits on the base board on one side. I've glued and screwed it in position with 4 screws per panel. Once the top rail and one bottom strip are in position, the pailings can be fitted in position and screwed top and bottom.