Now, to figure out how to mount the rear speakers... this was a tough one and I may end up changing the configuration a little bit. I have a ton of speaker cable rolled up inside of the seat base so I've left my options open. My original plan was to bring the speakers up next to the driver's head, but they weren't secure at all, too loud (even when turned down) and I was worried a kid would tear them off the side of the cabinet too easily.
After a lot of testing with my wife holding the speaker in different positions behind my head (thanks honey!), keeping the speakers lower to the ground and pointed upward really seemed to be the best configuration for the time being. In testing the 5.1 surround, the cars sounded like they were right behind you... slowly creeping up on the corners and then the screeching tires... While not ideal, it sounds great, looks pretty good and is as integrated with the cabinet as possible - while still being sturdy enough to survive a mild kid attack.
I grabbed two 1/2" wooden dowel rods and cut them to size. Then found some 1.125" corrugated black tubing, normally used for a washer's drain hose, to hide the speaker cable and dowel rods. I used 3/8" black heat-shrink tubing on both ends of the speaker cable to help it blend better.
We'll see where I end up going with them, but for now... acceptable.