My dad asked me to design and print him this. (I like that this was the first thing that sprang to mind when I said I bought a 3D printer.) It's a cap to go on you container of Comet (or similar sized container of cleaning powder) to keep the powder from losing all it's effectiveness as the chlorine evaporates out of it. (Link to Thingiverse page)
It's designed for the dimensions of one of the big, double-sized cans of Comet but it'll do the job on other brands too. Just don't force it on there or it'll take a beating to get it back off.
Like a jaunty beret, eh?