I'm with Mr. Whoopee here: a square is sufficient to make a chuck key.
That's kind of laughable coming from me though: I'm pretty much the poster child for TAS (tool acquisition syndrome). I tend to beg/borrow/buy/steal/make a new tool for the flimsiest reasons.
You'll almost certainly want to acquire some 5C collets and hex/square collet blocks eventually, if not a full blown indexer or dividing head. They are ubiquitous and not too dear if you buy used (or offshore). Even the offshore stuff is reasonable quality these days (though I still see "1 inch" collets that are really 25mm, and they all need deburring). It's usually wisest to just acquire the few sizes you KNOW you need, though, rather than "complete" sets. It won't take long to build up a decent collection if you hang out at enough machinist meetups.
One thing about indexing and orthogonal positioning that took me a while to learn: a grinding-vice, custom fixture, or even just a temporarily attached piece of scrap of convenient shape can often make positioning tractable (and accurate). Since we're usually removing material, it's not always obvious that ADDING something can often make the machining easier.
Joe Pi did a good job discussing indexing a few years ago:
-- note how his little indexing plate uses the "add something" idea. As he shows, dowel pins are GREAT for accurate positioning, too. Just drill and ream a few holes as needed and you can accomplish any geometrical positioning/alignment you can imagine.
Lastly, don't neglect extra stock in addition to vices/fixtures/attached-scrap. Not only does extra stock provide you something to hold onto while machining, if you think ahead you can use the shape of the stock to your advantage. You could, for example, turn the shaft of a chuck key out of square stock. Then to make the smaller square at the end, you could use the square extra stock to do the indexing (just rotating in the vice).