Here'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.
(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 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. (As in the photo where it's crammed on the can of Barkeeper's Friend at a jaunty angle... it took a hammer and screwdriver to get that back off)
Use the fastest and loosest settings because no one's coming over to admire your household cleanser cap anyway. (Well, except my dad, maybe.) Should print flat with no raft if your bed is leveled OK.