I am in the middle of doing something very much like this, but I had access to a yard of the exact same material that the rest of the kilt was made from. I also had made the over-apron a bit wide and the pleats a bit not-so-wide. So I can achieve the goal by adding 4 inches of pleats. It won't meet my hips exactly on the midline, but it won't be that far off. However, the project is substantial. It's nothing like starting from scratch, but it's still a mess 'o work.

In theory you could use more or less anything for the under-apron, but I think you'll be happier if you put in some research and try to find the best match you can for the cloth.

The Celtic Craft Center store is in Edinbugh, you could e-mail them pretty easily and ask.