This shelf bracket is designed to fit Ikea Burhult shelves, but will also fit any shelf with the same dimensions. The bracket's screw holes fit screws with a head width of 8.5 mm and thread width of 4 mm. You will need screw anchors to ensure that the shelf does not fall off the wall. To make 4 of these brackets, you need about 1 kg of filament. The bracket dimensions are 200.5 mm x 196.85 mm x 20.25 mm.
There is now a Version 2 that doesn't need 'support brackets.' You don't need to use 6 screws, 4 or 2 may be enough depending on how much weight you intend to put on the shelf.