Having had to use what I could find on the internet, my experience from previous sewing, and then spend time just 'fiddling' using what fabric I had and a lot of pins, I would advise that you just experiment - use any bit of cloth to fold and cut and see what happens - you can get lots of bits of stuff from the charity shops - assuming you have some close by.

Failing that you might have an old sheet - or you might even try out a thought on some spare wallpaper.

I have definitely settled into making reverse Kinguisse style kilts, about 7 yards, with no internal reinforcing just a sturdy belt.

I tend to use cotton, sewing the edges of the pleats, as the weather is so mild that I find even 50 percent wool too hot.

Perhaps I will feel the cold more if I get thinner.

As yet I have not cut out the back of the pleats, as I have been regularly returning kilts to straight cloth and remaking them smaller, So can't give any advice on the excess fabric.

However...

The purpose of cutting the pleats is to thin out the fell, to smooth the garment outline and I suspect reduce the insulation. I would tend to look at cutting the inner parts of the pleats, that is the cloth which is not on either the inner nor the outer faces. There would then be an unbroken layer of cloth both sides but not so many layers. On an ordinary pleat raw edges are left inside and need to be covered. It might be more satisfactory to retain more of the kilt material and - although it would mean hand sewing, form a smooth surface inside the kilt.