Ok, I've had time to process this, and look at the kilt while wearing it. Hopefully this makes sense because I can't do pictures and drawings.
I had been looking at the lower pocket of cargo pants, and thinking about putting something like it on the side of the under apron, about mid-thigh, or a patch pocket around the same place. I would have to, just as with the cargo pants pocket, bend forward a little to access the pocket.

The Wizard described a pouch, centered across the under apron, with an angled side making it easy to access from the side under the edge of the top apron. The angled side made sense, but that would mean the pouch is much higher on the under apron than what I was imagining with the cargo pocket. What he said made me completely rethink the pockets.

I instantly thought of the aprons I use when I'm working on ceramics, so I looked at them as a model. However, I also thought of something else that is a bit different. I have a few pullover jumpers, or what ever they're called, with a pouch on the front, except the sides are open, instead of the top. One of them has angled side openings and is very low on the jumper, so I have been looking at that as a model.

It seems like the tube-like pouch on the jumper could be extended all the way up to the waistband area on an under apron and partly supported by it. I guess only one side of the tube would need to be open, and only along the top end of that side with an angle or curve like in the front pocket of trousers. It also makes me wonder about a pouch that could be hung over the top of the underapron, and not even sewn to the kilt in the first place...