The .100 thickness may be ok.
I'm actually more concerned about metal fatiguing by the addition of a machined pocket with the danger of introducing stress risers.
If I do this, I'm going to make sure to use a milling cutter with a radiused corner so there is a radius all the way around the bottom of the pocket.
It's one thing to use proven factory parts but when you start modifying them, all bets are off.
