I remixed the design by Hanselcj, because I needed it to be stronger,  due to long shelves.
The original .stl was imported into Fusion 360, converted from mesh to a solid, then modified as needed.

I have long (1800 mm) shelves, and there will be some weight carried on the shelf.  The shelf goes above the table where my printers live, and its for storing unopened  rolls of filament.
The bracket is 2.5 times larger than the original  in all dimensions.
The dimensions: 40 mm x 40 mm, width 35mm
BOM: 
4 of  5x16 
or 
4 of 5x20 round head screws, self threading, which require a 12 mm csk hole to accommodate the large head.

to suit 16 mm thick MDF shelving material with Melamine covering.  The holes to accept these screws needs to have a pilot hole drilled thru the melamine coating to stop it cracking.

As a guide to those using this for shelving, it's also necessary with long shelves, to take some steps to reduce/eliminate sagging of the shelf.  I added bracing with natural pine lengths longitudinally placed on the underside of the shelf.  I also added long steel right angled brackets  bracing the vertical supports of the shelf.  I also added some lengths of 10 mm aluminium round tubing that fit corner to corner of the shelving frame to act as bracing to try to prevent wobbling.  It still wobbles, but less.

And still it sags under the weight of 27 1Kg boxed filament rolls.  I'll have to rethink the shelf support beams I guess (!)

Printing guidance:

Printer: Prusa Mk4 in Prusa Original Enclosure.

Material :  Prusament PC Blend  I used this material to provide greater strength. 

Nozzle: 0.4 mm Hardened steel (regular brass would also work)

Profile: Stock prusa slicer 0.2 Quality with tweaks as below:

Infill : 30% grid infill

Seam: Random

Speed: reduce infill speed to 90 mm/s and solid infill to 70 mm/s  to eliminate warping (Mk4)

Cooling:10% fixed