The missus of the house wanted more shelves in the house cabinets. Carpenter ask for alot of money. So i bought some compressed wood boards painted them in white and made some 3d printed supports. The shelves have alot of length so to avoid any bend i design these simple supports to use a L shaped or angle steel bar to reinforce. The steel bar in this orientation has alot of support strenght and doesn't bend easily.

I used 25mm Angle Steel bar, and the supports are fixed to the cabinet with normal M4 wood screws.

The other supports seen in the images are:

https://www.printables.com/model/291536-shelf-support-shelf-holder-shelf-pin-shelf-bracket

and

https://www.printables.com/model/470565-shelf-mounting-corners

I was able to make the shelves for 1/10 of the carpenter quotation. Probably not has good job or as seamless but it does the job very well.