The Nexstar SE does not have standard camera tripod interface, but three 3/8" threaded holes near the edge.
Therefore, I have to create a metal plate having one 3/8" threaded hole to connect to the tripod, and three 3/8" holes to connect with the base of the Nexstar mount.
Finding such a plate is pretty easy and at the same time, hard. We don't have to use a strong plate since it's just "the meat between the sandwich", and I believe even a plastic plate will do the job nicely. We will need a plate of around 9" diameter.
I placed a piece of paper over the base of the Nexstar plate, and use pencil to mark the holes. Then I stick that paper over the metal plate for drilling:
I drilled one more small hole beside the 3/8" screw, i.e. the guiding pin of the Gitzo:
Placing the plate between the original mounting plate and the tripod:
Fixed the Nexstar mount on top via those 3/8" holes:
Frankly, there are some vibrations but it's acceptable since this setup is only for visual purpose, photography is another story but then it's not a photographic mount afterall.
A bigger plate could be made so that the excess areas could be used as eyepiece holders by drilling 1.25" and 2" holes.