If I may,,,,

An under-apron Pocket is just a rectangle of fabric sewn onto the surface of the Under Apron.

A Side Pockets is, yes, similar to those in slacks.

But that is where any similarity ends.

I spent many hours and killed quite a few brain cells working out how to put my pockets into my Kilts without harming the look, hang, and swish of the Kilt.

A good Pocket in a Kilt is not just stuffing an extra flap of fabric in a pleat. There really is a bit of Engineering and trial & error involved.

How I do it is only one way. I sort of guard my construction method and keep it to myself.

You could always wait till later this year when "The Art of Contemporary Kiltmaking" comes out.