Here you can see the flanges that have had the main hole cut through. They are being aligned prior to temporarily screwing them together and then drilling the main bolt holes. Since this will be a precisely designed electronic device with networking compatibility, Paul thinks that 8 no. M12 bolts would be appropriate for holding the integration hemispheres together.
This was more difficult than it initially appears. To ensure that the integration sphere remains totally spherical, the flanges have to be attached to the outside edges of the sphere. The sphere cannot be attached to the face of the flanges as that would create an elongated non spherical geometry. This detail can be seen in the CAD drawing. The edges of the hole have to be angled (at 15 degrees in this case) so that an adhesive can be used to firmly adhere each hemisphere to each flange. A high modulus silicone sealant was used as the adhesive. This allows a firm bond with some flexibility at the MDF to polystyrene interface.