I also had to cut the face of the new mirror to make the two roughly line up..
Once you can get the two halves flat enough so that you are happy with how they will sit.. You need to drill holes (7-10mm) in any large flat areas to provide a better anchor in the surface.. Don't matter if the holes don't line up..
The more holes the stronger the bond..
Now before you can bond the new mirror to the modified cheetah.. You'll need to refit the cheetah to the car (don't re-fit glass yet) this is so you can line up the mirror on the car..
Now using engineering/construction resin.. Pump resin into every hole and apply an even coat over surfaces..
This stuff goes off quick and generates heat whilst it cures.. It can be sanded with a disc cutter after.. But remove any excess before it goes hard..
With both sides covered in resin.. Butt them up against one another.. And hold or clamp in place until cured (5 mins)
Once set I removed my cheetahs and left them laid down for 12 hours to ensure the resin had gone solid..
You get something like this