You just use the part with some well guides or however you want to drill the holes and glue it down. The 2nd photo shows everything you need, two of the easy_button boards, 4 screws like the ones used to hold the PCB down to the front of the case, a membrane that you will cut in half and trim so it fits around the standoffs and a couple of GB buttons. You'll notice I didn't tighten the screws down. That's because I'm not sure how many times I can do that with this print before the screw stops biting.
Obviously this won't work out with an SD cart if you're going that route as this interferes with the cart reader. I'm using Kite's SAIO so it's not an issue. I'm going to cut the front of an empty cart to accommodate.
https://www.dropbox.com/s/6zydv18me7l98 ... 1.STL?dl=0