My shelves were super wobbly, so I designed these to clamp in the corners and provide some rigidity. They work pretty well. Requires a drilled hole in the shelves, but I got mine out of the dumpster.
Required non-printed parts:
M3 heat-set insert (plus soldering iron)
M3 bolt (length depends on the shelf dimensions)
I used the following settings. Pretty much arbitrary.
Printer: Ender 3 V2
Filament: Recycled PETG
Layers: 0.12 mm
Infill: 30% Gyroid
The author marked this model as their own original creation.