There is more than one way to skin a cat sez my pa.
Really well done.
In my case, I put the extra balsa around the pinewood platform, drilled out the weight pockets and everything else between the wheels, then glued a 1/64th in. balsa on the bottom to make it flat. Weight went on top of the balsa. The huge pocket between the front and rear wheels remained hollow. I piled all the weight into the back. Because the front is so long, the fulcrum effect made the COM turn out right.
I cut the entire platform to slip into the lexon body, left the front end even, but pushed the rear end up into the Lexon body by almost an inch and cut off the extra Lexon, thus the car had a low profile, almost horizontally even at the top.
My yellow car was my first attempt, and I didn't follow all of my own building rules. As a result, it is at least 1/100th slower.
Your body looks the same as the toytech's. Where did you buy it?