Here's the thing - you want to make sure that the solid batten will keep the sail shape consistent. In other words - it needs to have similar stiffness/bendiness in the rights places.

The good news is that you can make the solid batten more bendy in specific places by carefully sanding on both sides.

The bad news is that you can't really add stiffness.

On the assumption that the solid batten is mostly as stiff as needed (usually toward the roach of the sail), try this -

Put the battens in the sail, normal tension, then pull the boat over on its side, set your downhaul at your minimum setting (just getting the wrinkles out). Put the solid batten in, and see if you can get the sail shape to consistent through the area of the solid batten. Try adjusting how tight it is in the pocket, but avoid jamming it. Pull the batten out and sand in the area that needs bendiness, try again.