I repeated this process three or four more times, each time removing some of the (not very sturdy) foam insulation and filling it in with more bondo..
Each time between filing and the next coat, you want to see how your fitment is so that you can apply more/less bondo in the right spots.
At this point, I had a rough frame, but obviously it's not going to sit in there by itself, so I had to make some mounts for it. I bent up some pieces of metal to mold into it that would fit the right places on the dash to hold the bezel securely. Here you can see how the mounts are configured. They are secured into the bezel by putting them down into wet bondo and letting it set around the metal in place.