First I have to confess to not making structured kilts, but I know how to, and to do lots of other sewing too.

The whole raison d'etre for the internal reinforcement is for it to be shaped to the body beneath and provide a foundation for the external layers.

The rigidity of the material is why it was chosen, so moving the shaping to the edges is not going to provide the three dimensional adaption to the curves of your anatomy - not unless you are rather unlucky.

Do a Grant - rejoice in you rear (this will be understood if you have delved into the 'Off Topic' section) and put in the shaping within the canvas as required to round out the reinforcing layers.

Anne the Pleater