A small parametric shelf. The dimensions can be entered in the variables in the onshape design. It's not very sturdy, but it should be able to hold small weights. This version is not stackable.

Improved version with better stability, using the suggestion by @BikeCyclist.

I used ABS with 4 walls with 25% infill and no supports, but I think 3 walls and 20% would be enough.