Might be easier to move the mirror back instead of cuting new holes and filling old for the focuser and diagonal.
Simple make some 100-130mm brackets to attach to the original mounting points on the cell and tube.
If you slotted the mounting holes in the brackets you could make it adjustable to get the right position.
Once you have the desired length make a shroud to go around the gap from the cell to tube.
Even easier make a truss tube :simple to adjust the truss lengths
Mark