I decided to buy an ITA SAO bag and then sew the simple add-on inlay for it. I started like I would do welt pockets, but doing 44 of them simultaniously. You start with the two "good" sides facing each other and sew them together with the square opening you want to achieve. After sewing, I've cut the black cotton canvas material diagonally, so I could then cut a crossed hole right up to the corners and fold the flaps through the hole and basically turn it around. There are better and worse results when doing it 44 times, but this will probably hidden well enough with a lot of SAOs anyways.

I will probably cut up the wires and do the connectors next and then sew everything together.