To build this, I used about 48 inches of rolled heavy gauge wire. First by measuring each of the dimensions of the tablet and the visor. Straighten the wire, then starting from the middle of the wire, use each of the measurements from the visor first, making bends to create the hook for the visor, then make the 90 degree bend from which the tablet will be suspended. Next, go to the opposite ends, and using the measurements of the tablet, make the appropriate bends allowing for the thickness of the tablet. These bends may need to be tweaked to make the tablet level when in the holder.
This one was made with a 7" Kindle in mind. For a larger tablet, more wire will be necessary.
Step 1: Finished Tablet Holder
This is the finished holder with the tablet in place. Remember to compute safely and don't use your tablet while driving. Lastly, rolling the tips of the wire back on itself will keep the wire from scratching the display of the tablet.