Darn. A suggestion: If you repeat this, place a dam of sandwich wrap on top of the wood, a sheet just a bit bigger in perimeter than the inside perimeter of the shell of the car. You want it so that the load, when added, will depress all but the sides of the wrapping and form a shallow 'pan'. Then, add your glue. Personally, I would add some undiluted wood glue to form a block of the material against the sandwich wrap, but not so much that it shows as a pool near the top. Later, say after one full day of drying, maybe more, dribble a more diluted mixture to fix the top grains of 'coal'. Later, you should be able to remove the coal load, with a little luck, if you ever wish to for some reason.