I've always put the horn plates in boiling water.
Let it boil till bendable & pliable.

Quickly set them it place on the stock and you can press it into shape with simple hand pressure. But the hot matr'l isn't too pleasent to handle so some weight is the usual method to push the plate into it's new shape.

A partially filled shot bag works well on the face of the plate. It won't take a whole lot of weight if you get it on there while the horn is pliable.
It cools and stays in the new shape.

When the curve is so slight that you are trying to produce in the center of the plate,,you sometimes are farther ahead to slightly,,very slightly shim the ends of the stock face to produce a slightly exaggerated curve to the plate.
The same thickness of the business card that the plate needs as a curve in the center can be built up at both toe and heel. Any hard edge must be shaved or sanded off of the card or that will show when the soft horn is pressed into it.
The whole idea is to create a slightly deeper curve in the center by a few .000 and then the attachment screws will draw the plate down nice and tight.

Over kill perhaps,,but it assures that there is no gap when it's all done under the plate at the mid section.

If you use clamps and rubber bands and that sort of thing to pull it into shape, the smaller area contact pressure points of those things can often imprint into the rear face
of the horn where they contact and that leaves their mark when cooled as well.
If you use clamps or bands, spread their holding power over the area with something like felt or leather pads so you don't leave an imprint.