Greetings Patrick. There are several ways to do this. My method is just one of many.
I place the license plate on top of the box and mark with pencil where the mounting holes are. I use standard license plate mounting bolts and screws to affix it. Before attaching it I cut a hole in the top of the box well within the mounting hole area, so that there is a large opening beneath the license plate. This allows the plate to vibrate properly and give the git that special sound.
I run the stick through about 1/4" under the license plate (or more) and use a single piece of wood at the bridge to form a brace between the license plate and stick-through, allowing the strings to vibrate through the bridge and into the neck while leaving the rest of the license plate free to vibrate of its own and provide that unique sound. This is especially audible if you use piezo pickups but works even with mag pickups. What type of pickup you use is totally up to you; you'll get good sound with just about anything (albeit widely varied). I prefer flat pickups as they can be attached directly to the license plate and will themselves vibrate under the strings as the license plate vibrates. The result is fantastic.
Other than that, nothing else that I can think of that is special. Be sure to ground your strings of course. Here is a video of a LPG I made and captured video of an associate playing it. In this model I used triple piezos trapped between the bridge and license plate, and up by the neck.