I have a pretty big medicine cabinet, and use something similar to this to keep stuff corralled in there.
drawer organizersIf your shelves are at least 4" deep and 12" wide, the long, skinny one of these will work.
The ones linked to aren't the exact ones I have; mine are ancient and probably no longer made, and they didn't have honeycombs or anything. But they're the same idea--they come flat and you fold up the sides to make the box.
Mine are completely clear, while these are frosted. And on mine, anyway, the plastic is thin enough that you can cut it if you need to make a notch or lower one of the sides. These look like they could but cut, too.
One advantage they have is there's a bottom on there, so if something leaks it might not make such a big mess. I've been using mine for 12 years of fulltiming and they're still working. Actually, one of them has split at the front seam, where the front wall meets the floor of the box (I think I had too much stuff in it), and I haven't even taped it--it still works fine for holding everything in.
