I am going to be doing mine soon. If find the GSG grip safety hard to push in.
One way that I read about is as follows.
Drill a small hole in the top of the mainspring housing where it butts up to the bottom of the grip safety. Drill another hole in the bottom of the grip safety lined up with the M/S hole when the grip safety is in.
Size the holes & make them just long enough for a metal rod(pin) to go in both.
When the M/S housing is put back in, the pin in it will go into the matching hole in the bottom of the grip safety & hold it in.
There are other ways but this way is reversible.