I needed small brackets for mounting a shelf over my toilet. To avoid hitting the pipes during drilling I went for side brackets mounted on the left and right wall. Mounted on brick walls with 5 mm wall plugs (Dübel) the brackets feel really secure holding a 80 cm board. I didn't design for securing the board with screws as the board is a tight fit between the side walls. If needed it would easily possible to adapt the design for more screws into the wall or board.

OpenSCAD design is included and can easily be adapted.

I printed the brackets smaller then the board so they are nearly invisible below it.

Print Settings

Printer Brand:

Creality

Printer: 

Ender 3

Rafts:

No

Supports: 

No

Resolution:

0.24

Infill: 

15%

Filament: ICE PLA White

Notes:

I used 4 perimeters and a low infill of 15% which gave me a good result.
Print time was around 8 h (not optimized).

4 bottom and top layers (top doesn't matter)

Easy print, most of the time you will need 2. One for each side of the shelf :). Rotate z by 45° for max. length on your print plate.

With my print one of the corners lifted a little bit. This part was mounted facing the back wall and is therefore not visible. Brim might help to avoid this.

Adapt filament color to wall and board.

Post-Printing

Finishing

I slightly trimmed and sanded the corners to remove a little elephant foot.

How I Designed This

Most basic design: Create cube, remove cylinder, add holes for screws.

I love OpenSCAD for quick designs like this.