Ikea Ivar shelfs bracket. Works pretty well when using the settings in the provided 3mf setup (50% infill and  6 perimeters ). Untested on heavy loads , use at your own risk.  4 point load sharing concept, not usable at the crossbar height (one vertical position option lost). 

Features:

• 4 pcs. needed for 1st shelf  (2 more for additional shelf on next rack) 
• Holds 2  shelves at once (when racks strung together)
• Quick remove/reposition lever slots on both sides (from Version 1.1.)
• 4 point weight sharing