Well, if I were to attempt, I would most likely try make some sort of mounting frame to support it from the sides. If I were to get fancy, I would cut wood frame, or even go for an all out 3d print of a frame. But because I don't care what the insides of the case look like, I would probably get a stick of that plumbers clay expoxy and wad around the sides of the retrousb to hold it in place.
brihyn's idea sounds better. I would attempt to solder wires to the back of the original in-case connector, and solder terminal pins to the other side of those pins, then use pliers to push those pins into the RetroUSB and make a connection.