A simple bracket for making shelves from lengths of 40mm PVC pipe, a very cheap and available material usually used in plumbing.

The brackets are 45 degrees, with 6 mounting screws to maximise strength, I ended up putting about 12kg on each pair of brackets with no problem. Simply cut two lengths pipe (mine are 70mm and hold 10 filament spools), use the provided pipe caps with some self tapping screws to make a shelf deck. This can then be hung on top of the brackets which have been attached to the walls with 4.5mm wall screws. You can use the provided hole alignment jig to drill 6mm holes.

The bracket is weight bearing so I would not use PLA, PETG works well, and I am sure ABS/ASA and other more mechanical filaments would work too. 

The pipe cap can be any material.