If you decide to drill the bubble you have, you'll need to clamp the bubble to the base plate, assuming its drilled, and then transfer the holes to be drilled to the bubble using a transfer punch. Same with any other hole for air doors, etc. A transfer punch for the hole size in the base plate is the only way to go. If you need any other photos, just let me know. I've got plenty but I'm also willing to take mine apart.
Terry