I liked the idea and versitility of the horizontal multiboard as a shelf but I wanted to charge various items on it some of which could roll off. So I made a little border in TinkerCAD and then merged them in the slicer. I have no plans to make different sizes at the moment. If I have a need for it I will but otherwise this is it. I'm sure someone that knows OpenSCAD could work out a parametric version of this which would be great but that's beyond my abilities right now.

I'd suggest splitting the model into objects and setting the tile seams to random as recommended by Jonathan @KeepMaking for added strength. Bambu Studio gives warnings so I was unable to upload it that way but it does print just fine.

Brackets are here and there's instructions on mounting on that page.