I have now mounted a Vectrex inside the cabinet.  I have arranged it so that with the removal of just two screws the panel holding the Vectrex can just swing to a horizontal positon resting on a wooden dowel on the inside of each cabinet side. The picture shows the rear of the cabinet.

Note, there is an original  speaker positioned at the top of the cabinet - I will eventually connect the speaker to an amplifier which in turn I will connect direct to a pre amplification point on the Vectrex PCB (should remove some of that infamous buzz). Beyond the speaker (at the front of the cabinet) there is also space to put a back light  for the marquee.