And finally, before you drill that hole, are you sure you need it?
I ask because once at work I was told to drill holes in steel doors
for a similar peephole.
I found the doors were all solid, but - next to a window. (like a
motel room setup)
I couldn't see any point in the peephole, it didn't seem to add any
security to what was already there. Of course my boss told me to do
it anyway, so I did, but at least I raised the issue first.