There was a thread
here recently asking about a GSO 12" newt. I think this post will be relevant to the 12". I have a GSO 8" and had issues with the scope not holding collimation. The problem is in the primary cell design. What works for smaller scopes doesn't necessarily scale up very well as far as weight is concerned. My 5" Celestron had the same cell design and never had an issue. It worked fine for a small lighter mirror. The mirror in this 8" newt pretty much goes where it decides to go when the scope is pointing in various part of the sky and the so called locking knobs are useless.
The first thing I did was to replace the collimation knobs and locking knobs with stainless steel Hex screws. I also drilled and taped the cell to move the locking screws right next to the collimation ones. The original locking knobs were half way in between each pair of collimation knobs and didn't do much. I also counter-sunk the tips of the locking screws inside the mirror cell so they also stop the cell from rotating of moving sideways when it's locked.
Next thing was to modify the retaining clips. I bought the scope second hand and the mirror was siliconed to the cell with clips removed. The problem with the clips is that they are rubber and they pinch the mirror down against the cell. When they're tight enough to hold the mirror they pinch the optics. When they're just right as not to pinch the optics then the mirror is too loose for its own weight and shifts sideways and even tilts.
So I changed the mounting based on what a few others do on the web. I liked the protostar cells system and that's what I tried to replicate here. The finished product looks very simple but it was a fair bit of stuffing around and trial and error to make it all fit in the annulus between the mirror glass and the tube. There is really not much room to spare in there. The concept is a no brainer. The two original screws were cut down and small alu brackets (light color) hold the mirror centered. They hold the weight. The black brackets are the retaining clip. So they just stop the mirror from sliding up. There's a bit of rubber to make contact with the edge of the glass but that's it. There isn't any pressure. Everything is adjustable.
Now I can move the whole tube in different positions and the collimation holds. I also have no pinched optics, so best of both worlds. I assume that could be easily replicated on a 12" providing they use the same cell design or similar to the 8". Probably easier as there will be more room to play with.
