Cheese shelf brackets! The most unstable looking, but in reality, very sturdy shelf.

The brackets are 190x50x190 mm. They are perfect for a 20 cm or 8 inch wide board. You can freely choose the length and thickness of your board.
You can either screw or glue the holder onto your wall. The screw holes for the wall mount are 5.5mm wide and require countersunk screw heads.
The screw holes for mounting the shelf onto the bracket are 4 mm wide. Make sure to choose a screw size that will not stick out of the top of your board.

Instructions
Print the parts
Mark the screw hole position with the holder onto your wall
Drill the holes and place dowels
Mount the holders
Insert the shelf brackets from the top
Place your board
Screw the board in place with three screws for each bracket though the screw holes.

My shelf, printed in normal PLA, the bracket in default settings and the holder solid, resulted in at least 80KG of weight support. I stood on it. This test does not guarantee that outcome and I highly advise against standing on these shelves or performing any tests that could result in injury. They are very sturdy, but since I do not know your printer, material and settings, you can only treat this number as a reference.