I added a few shelves in our 2350 bathroom. I used deep-thread, deep-bite wood screws, but I pre-drilled the holes large enough so the screws would not crumble the tempered hard board. The holes are drilled a tiny bit larger than the inside diameter of the threads on the screws.
As far as the shelving I installed........
We roll up and store our bath towels in this rack mounted above the toilet. It works well. The towels are pressure-fit and don't fall out. The towels in the picture don't reflect actual usage.

I made a toothbrush, toothpaste, and hand soap shelf made from the cabinet base door that I no longer needed after my galley cabinet reconfiguration.
