take a look at my other designs https://www.thingiverse.com/mg_evolution/designs
The test piece goes into the foam board body to see if the print will fit your application
I have made a Nutball based on the flight test online plans.
I made a Brushless motor mount for the swappable body that will allow my 20amp ESC through it and space to mount a LED at the front bottom.
FT Swappable brushless Motor Mount Round.stl is a single print to allow better airflow to the body of the plane.
I used 5mm foam board and have based the sizes around this.
The hole size is 45mm wide x 31mm high,
There is a lip at the top to secure the nutball to the motor mount.
The smaller prints are guides for holes in the foam board, this will allow fixing the parts together with 2 x M3 screw and nut.
The screw head supports for the foam can be found here from my Skyhunter design:
If anyone has another hole size required and different size foam I can adjust this and post it up on request.