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