1. most likely some form of epoxy..
2. there is no circuit board, it is just a switch. He basically bored a 1 inch hole using a hole bore, and then bought a lamptron vandel switch which he wired manually.
3. seems he took a pci usb holder cable... unscrewed it off the pci plate, and then screwed it on behind, before epoxying the face plate.
I've done that many times, cannibalizing those accessories that come with the motherboard.
I can't zoom the picture, but I'm speculating that there is as lip on the chassis in the largest top-side surface. There may be an interference fit with the chassis at the bottom corners. You could install a snug-fitting panel on that chassis with blobs of Pit Crew adhesive where it can't be seen, and that would be a firm and reliable construction. The stuff dries to a semi-hard, translucent rubbery texture and adheres to any surface. You need a knife to cut and scrap it off, or you'd have to loosen it with a firm grip from some needle-nose pliers.
If the face plate is Lexan or similar plastic, anyone could install tabs with the specialized glue to fuse them in place, and then secure somehow on the inner side of the chassis. Apparently, the project avoids any exposure of screws or fittings on the outside -- I can't say for sure.