I needed some support plates for some bookcases, so this is what I ended up with, you place them at the top inside corner of your bookcase or picture frame.
There are 2 holes for some small nails, just to make it sit tight and dont move, then you can use the main hole as a drill template, and for the fixing to the wall.

The corner plate is 7mm. thick.

i have added a .STL with 4 pieces for printing, if needed.

Anycubic

All-metal Mega

No

No

200

20

Speed 50mm/s.
No support.
No raft.
infill 15-20%