Tatters are such smart and creative people, always thinking of new uses for everyday objects! When I asked for help on caps for the tiny crochet hooks, I got some terrific suggestions. Sarah suggested the caps from medical needles. I work in a hospital, for heaven's sake, why didn't I think of that? I picked up a few at work last night (the caps themselves stay clean because we don't recap needles, but they would go in the trash anyway once the needles are used).
Martha suggested cutting up coffee stirrers. These have a different diameter from the needle caps, so I thought I'd give them a try too. I picked up a couple of them at work, too (they didn't give us raises for four years in a row, they can spare me two coffee stirrers). I didn't bother taking pictures of them because I figured everyone knows what a coffee stirrer looks like.
Muskaan suggested wrapping my homemade caps in tape to reinforce them, so I did that with both the needle caps and the stirrers, after cutting the stirrers in half. Again, I used stuff from work-- the nurses use these colored tapes to label their IV lines and keep track of what's going where.
I brought them home and tried them out, and they work great! The needle cap fits perfectly on the crochet hook that has a plastic handle. The ones with metal handles are thinner, and the coffee stirrers work on those; cutting them in half made for a good length so that when they reach the part of the handle that's thick enough to prevent them sliding down any further, the tip of the hook will not be sticking out (I also sort of closed them by twisting the tape to a point at one end).
Success! They're not beautiful, but they work, and that's all I care about. Thank you, everyone, for your wonderful ideas!