I've seen these around and bought a couple. I am in the middle of Early January Fly-Fishing Angst Mode, when the only fishable water is about two miles of the West Branch of the Farmington, right where it comes out of the dam, and my idea of a good time is watching Davy Wotton videos while curled up in a blanket.
It's a slender thing, measuring 3/4 inch thick and 3 1/2 x 7 inches in area. It fits into a Fishpond chest pack. You could probably cram three or four in.
There are slots for 168 flies, but unless these flies are tiny and you want to use tweezers to get them in and out, I don't think you'll get every slot filled.
Hard to say how it works until I get out and try it, but I think it will allow me to consolidate all the nymphs in one (or two or three) boxes with a lot less bulk involved.