I've made it.
Luckily, if I close the bit holder with the mag glass in it and and "shake" the knife, it does not move - so I guess I don't need a scratch protection sheet?
I've made one mistake: Obviously I was too forcefully and a little part of the plastic ring broke (see the photos attached). I've glued it and it seems to be stable. There are some glue rests on the plastic as well as the glass. Do you think it's ok to try to remove that glue with spiritus, or do you see the risk that the glass blurs or something similar? Is the magnifier itself glass oder also plastic?