When you drill into this block you will find wood/plywood.
Sometimes this area can have voids in the glass,wood and bonding agent.
I suggest that if you find a void you fill it with resin or epoxy before you fix any fittings as this is going to take alot of strain.
When it is dry,drill the pilot holes for the screws you are useing AND then countersink the pilot holes to the same width as the screw shank.
What this does is stops the thread of the self tapping screw from lifting the gel around the screw out,Horrid if you can see it showing outside the fitting.
Then, fix the screws in with Araldite or Epoxy for a firm job.