A rear kilt pocket is actually easier to make, but far harder to make right, than a side pocket.

Usually they produce a visible bulge when you put your wallet in. Then the bulge causes the pleats to hang funny.

If you do a pocket similar to some manufacturers you simply slice a line through the back of the kilt and the take a pre-made tube and serge one end of it to the edges of the hole. Once in you stitch up the bottom of the tube.

Sounds simple doesn't it?

In truth to look like anything else than a hole in the back of your kilt you should do a welted pocket. Now a welted pocket is the kind found in dress slacks. Even experienced sewers hate making a welted pocket.

Add to the already complex process the fact that you have to get a welt over and covering all the layers of fabric in the Fell and you usually end up with a thick bulgy mess.

I know this from experience. My first pockets we horrible.