This is a simple bracket that I went ahead with and used 3DKitchen's tutorial on topology optimization for guidance. I set the weight to something like 5,000N, but I doubt this will hold that.

check out his tutorial here!

The idea of this model is to print 4 of them (You should have 2 normal, and 2 mirrored), and should be able to lay a board on the shelf you have made!