Simple 6'' x 6'' bracket for a small shelf or something along that line.
The diagonal bar has holes to access the internal mounting points with a long-ish screw driver.
I included the .rsdoc file, in case you want to adapt the bracket to your specific needs.
I use two of this (printed in PETG) to hold a 6''x12''x0.75'' poplar board attached to drywall. On top of it i keep 3 spools of filament (~6.5 lbs), and everything feels sturdy.